Danielle L. Bishop, PharmD, BCPP

Ms. H, age 42, was given a diagnosis of bipolar disorder 10 years ago and has been taking carbamazepine, 1,200 mg/d, and olanzapine, 10 mg/d, for the past 2 years. She has not experienced a mood episode while on this regimen, and her car­bamazepine level was 9.2 μg/mL 6 months ago. The only adverse effect she experienced was weight gain of approximately 10 lb. Ms. H takes a calcium supplement, but no other medications.

Ms. H reports to her psychiatrist that, for the past few days, she has been feeling nau­seated, fatigued, and dizzy, but has contin­ued taking her medications as prescribed. Her carbamazepine level is found to be 13.1 μg/mL. Ms. H states she has not started any new medications or supplements; her serum creatinine and liver function test results are within normal limits.

Upon further questioning, Ms. H says that an upper respiratory infection has been “going around her office,” so she increased her vitamin C intake by drinking 2 glasses of grapefruit juice a day (she doesn’t like orange juice). She has heard grapefruit juice can cause problems with some drugs so she is careful not to drink it at the same time she takes her medications. Her psychiatrist recognizes there may be a drug interaction involved, and recommends Ms. H hold her carbamazepine for 1 day and not consume any more grapefruit juice. A few days later, she reports feeling much better during a fol­low-up call and she makes an appointment to have her carbamazepine level rechecked in a we

Although grapefruit products are high in vitamins and low in calories, they can be associated with potentially serious drug interactions. The interaction between grapefruit juice and the calcium channel blocker felodipine was discovered inad­vertently >20 years ago; since that time, possible interactions with >85 medica­tions have been identified.1 Interactions with grapefruit products are complicated because, although most result in increased drug exposure, reduced exposure of the medication also can occur. Additionally, the degree and clinical significance of the interaction varies among individuals and from one drug to another.

Mechanism of action
Most interactions with grapefruit products are thought to result from the inhibition of intestinal cytochrome P450 3A4 (CYP3A4). CYP3A4 is involved in the metabolism of numerous drugs, and is the most abun­dant cytochrome P450 enzyme in the liver and epithelial cells lining the intestine.² Although hepatic CYP3A4 is thought to be minimally affected by grapefruit, inhi­bition of intestinal CYP3A4 can result in an overall increase in bioavailability of medications that are substrates and raise the risk of potential toxicity.³ Grapefruit contains various chemicals collectively known as furanocoumarins, which are largely responsible for inhibition of intes­tinal CYP3A4.⁴ Additionally, Seville oranges and the pomelo (a large, sweet grapefruit-like citrus fruit) also contain furanocoumarins and could have a simi­lar effect, warranting caution with certain medications.⁵

Inhibition of CYP3A4 by furanocouma­rins cannot be reversed, and new enzymes must be synthesized to return to the pre­vious level of function.⁶ Therefore, drug interactions resulting from CYP3A4 inhi­bition can last for as long as 72 hours after ingesting grapefruit products.⁷ Separating consumption of grapefruit products and medication administration will not help manage this interaction.

Grapefruit products also could affect drug disposition through effects on vari­ous drug transporters. Decreased systemic exposure to certain medications could occur through grapefruit’s inhibition of organic anion-transporting polypeptides (OATPs). OATPs form a family of drug uptake transporters found in the intestine, liver, kidney, and brain.8 For drugs that are substrates of OATPs, grapefruit’s inhibition of this transporter can result in decreased absorption and a resulting decrease in effi­cacy. Flavanoids in grapefruit, such as nar­ingin, inhibit OATPs, which is competitive in nature.9 Unlike the irreversible inhibi­tion of CYP3A4 by furanocoumarins, flava­noids effects on OATPs have been shown to decrease within 4 hours.¹⁰

No psychotropic medications have been identified as being susceptible to this interaction, but for those medications affected—including fexofenadine and levothyroxine—separating consumption of grapefruit and medication administra­tion by 4 hours could avoid this interac­tion.¹¹ Additional data indicate that orange juice and apple juice could have similar effects on OATPs.¹²

Perhaps the most well-known drug transporter, P-glycoprotein is part of the multidrug-resistant subfamily of trans­porters. It is located throughout the body, including in the intestine, kidneys, liver, and blood-brain barrier. P-glycoprotein acts as an export pump to decrease the cellular concentration of many different drug substrates, and many agents can alter P-glycoprotein’s expression or function.

Small changes in P-glycoprotein’s activ­ity can result in substantial changes in the disposition of substrates, which can include certain antineoplastics and antiretrovi­rals. Most reports have found grapefruit juice inhibits P-glycoprotein-mediated efflux; however, there also are reports of transporter activation.⁶ Additionally, P-glycoprotein and CYP3A4 share many substrates, so it can be difficult to iso­late the contribution of P-glycoprotein to grapefruit−drug interactions.¹³ The effect of grapefruit on P-glycoprotein activity has been difficult to fully elucidate; more studies are needed.

Grapefruit consumption and its effect
Drug interactions can occur by consum­ing commercially produced grapefruit juice and juice from concentrate, as well as freshly squeezed juice and grapefruit seg­ments.¹⁴ CYP3A4-inhibiting furanocouma­rins also have been isolated in grapefruit peel; it is not known, however, whether items made from peel (marmalade, can­died peel) contain concentrations high enough to pose a risk of a drug interac­tion.¹⁴ Contributing to the unpredictabil­ity of grapefruit-drug interactions, the amount or concentration of furanocou­marins can vary among grapefruit prod­ucts and brands.¹⁵ This variability can be influenced by the variety or maturity of the fruit and the fruit’s exposure to envi­ronmental stress.⁴

The frequency of consuming a grape­fruit product can influence the degree of a drug interaction. In general, consum­ing one 8-oz glass of grapefruit juice or the segments from a whole grapefruit is enough to alter a susceptible drug’s phar­macokinetics.¹⁴ Regular grapefruit product consumption, however, can result in an overall greater effect.^16,17

Lilja et al¹⁶ conducted a randomized, 4-phase, crossover study to look at the effect of grapefruit juice dose on kinetics of triazolam. Grapefruit juice was found to increase the mean area under the concentration-time curve (AUC) of tri­azolam compared with water, but no dif­ference was found between single glasses of normal-strength and double-strength grapefruit juice. However, repeated consumption of double-strength grape­fruit juice (200 mL, 3 times/d for 3 days) increased triazolam’s mean AUC by 143%, compared with an increase of 49% with just a single 200-mL glass of double-strength juice.¹⁶ Recurrent consumption of grapefruit juice (8 oz, 3 times/d for 6 days) also was found to increase the kinetics of the antihypertensive felodipine more than a single glass of grapefruit juice.¹⁷

Clinical consequences of an interaction between a drug and grapefruit can be difficult to predict. Drug concentration changes caused by a grapefruit interac­tion could vary based on interindividual differences. The amount and activity of intestinal CYP3A4 can vary from per­son to person, and can be influenced by genetic polymorphisms in addition to race, age, and environmental variables.¹⁸ Interindividual sensitivity to a change in a drug’s concentration also will differ, and patient-specific factors, such as concomi­tant drugs or diseases, could influence the likelihood of harm.

Interactions with grapefruit products are not necessarily a “class effect,” and specific drugs within a therapeutic cat­egory can be affected (although others might not). Several drug-specific charac­teristics can help gauge the risk of a clini­cally relevant interaction with grapefruit, including:  
   • metabolism through CYP3A4  
   • low bioavailability  
   • oral administration  
   • a narrow therapeutic index.¹

For drugs with low bioavailability because of first-pass metabolism, grape­fruit’s inhibition of intestinal CYP3A4 can result in a greater relative increase in plasma concentrations compared with a drug with high bioavailability.¹⁹

For example, an increase in bioavailabil­ity from 5% to 10% will result in a much larger increase in AUC and overall clinical exposure compared with an increase from 85% to 90% even though both represent an absolute increase of 5%. Although a drug does not have to have low oral bioavail­ability for an interaction to occur, lower bioavailability means that a drug has a higher likelihood of causing a significant interaction because of altered pharmaco­kinetics. Of note, injectable medications will not interact with grapefruit because metabolism through intestinal CYP3A4 is bypassed and grapefruit does not signifi­cantly inhibit hepatic CYP3A4.

Although grapefruit products could alter the pharmacokinetics of susceptible drugs, those changes might not be associated with adverse effects. Therefore, a factor to con­sider in evaluating a potential interaction with grapefruit is the drug’s therapeutic index and its risk of serious adverse effects. Drugs with a narrow therapeutic index are of particular concern because a significant increase in therapeutic or adverse effects could result from a relatively small increase in the drug’s concentration.⁷

Which medications are affected?
Among medications identified as interact­ing with grapefruit, some cardiovascular agents and several of the HMG-CoA reduc­tase inhibitors (statins) have garnered the most attention. However, grapefruit also can affect the metabolism of several psy­chotropic medications through inhibition of intestinal CYP3A4 (Table).^16,20-35 Prescribing information for some drugs warns against consuming grapefruit while using the medication. Among CNS agents, buspirone, carbamazepine, lurasidone, pimozide, triazolam, and oral midazolam all have such warnings in their product labeling.

Buspirone currently is not recommended with “large quantities of grapefruit juice.”²⁰ A randomized, 2-phase crossover study looking at the effects of grapefruit juice on buspirone’s pharmacokinetics found that double-strength grapefruit juice (200 mL, administered 3 times/d for 3 days) resulted in a 9.2-fold increase in mean AUC and a 4.3-fold increase in mean Cmax after a single 10-mg buspirone dose.22 Highlighting the wide interindivid­ual variability seen with drug-grapefruit interactions, the increase found in buspi­rone’s AUC ranged from 3-fold to 20-fold among study participants.²²

Carbamazepine product labeling lists grapefruit juice as a CYP3A4 inhibi­tor that is expected to or has been found to increase plasma levels of the drug.²⁰ Carbamazepine’s bioavailability is influ­enced by intestinal CYP3A4 activity; in a randomized, 2-phase crossover study of 10 patients with epilepsy, grapefruit juice was found to increase AUC of carbamaze­pine by 41% and Cmax by 40%.^23,36

Lurasidone and pimozide, although not specifically studied, have product labels that recommend avoiding grapefruit juice because it could inhibit metabolism of these agents by CYP3A4.²⁰ Of particular concern is the potential for elevated levels of pimozide to increase the risk of adverse cardiovascular effects including QT inter­val prolongation.¹⁹

Midazolam. Although grapefruit juice does not affect the disposition of IV mid­azolam, pretreatment with grapefruit juice was found to increase the AUC and Cmax of oral midazolam by 52% and 56%, respectively.³⁰
 

Other considerations in drug-grapefruit interactions
Cautionary statements about a possible interaction with grapefruit juice for many other psychotropics can be found in com­monly used drug information references or online sources. If you are concerned about a possible interaction and avoiding grape­fruit products is not feasible, consider a different medication in the same class.

However, you also should consider the level of evidence supporting any pur­ported interaction. Several psychotropic agents do have studies or case reports sup­porting an interaction with grapefruit, but cautionary statements could be based on theoretical concerns because of a medica­tion’s bioavailability, metabolic pathway, and concern for increased adverse events related to higher drug concentrations. Adding to the confusion, cautionary state­ments can be found about medications, such as clozapine, that have not been shown to have an interaction with grape­fruit juice when studied.

With many of the drugs that have a reported or theoretical interaction with grapefruit, data are inconsistent as to whether the resulting interaction will be clinically relevant. A number of variables relating to the individual patient, grape­fruit product, or particular drug can play a role in the significance of an interaction. Additionally, effects on drug disposition can last for a few days after consuming a grapefruit product.

Keep alert to situations of increased risk
Recall that the case patient, Ms. H, pre­sented with an elevated carbamazepine level and suffered resulting adverse effects because of an interaction between the drug and grapefruit juice. Although Ms. H was careful to separate intake of grapefruit juice from carbamazepine administra­tion, grapefruit’s inhibition of intestinal CYP3A4 still was present, leading to the interaction.

It is important for health care profes­sionals to recognize this potential risk and to advise patients regarding possible inter­actions between medications and grape­fruit products.

Related Resources
• U.S. Food and Drug Administration. Grapefruit juice and medicine may not mix. http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm292276.htm.
• Hanley MJ, Cancalon P, Widmer WW, et al. The ef­fect of grapefruit juice on drug disposition. Expert Opin Drug Metab Toxicol. 2011;7(3):267-286.
• Andrade C. Fruit juice, organic anion transporting polypeptides, and drug interactions in psychiatry. J Clin Psychiatry. 2014;75(11):e1323-e1325.
 

Drug Brand Names
Alprazolam • Xanax                                     Lurasidone • Latuda
Buspirone • BuSpar                                     Midazolam • Versed
Carbamazepine • Tegretol                            Methadone • Dolophine
Clomipramine • Anafranil                             Nefazodone • Serzone
Clozapine • Clozaril                                     Olanzapine • Zyprexa
Diazepam • Valium                                     Pimozide • Orap
Felodipine • Plendil                                     Quetiapine • Seroquel
Fexofenadine • Allegra                                Sertraline • Zoloft
Fluoxetine • Prozac                                    Trazodone • Desyrel
Fluvoxamine • Luvox                                  Triazolam • Halcion
Haloperidol • Haldol                                   Ziprasidone • Geodon
Levothyroxine • Levoxyl, Synthroid   

Disclosure
The author reports no financial relationships with any company whose products are mentioned in this article or with manufacturers of competing products.

Ms. H, age 42, was given a diagnosis of bipolar disorder 10 years ago and has been taking carbamazepine, 1,200 mg/d, and olanzapine, 10 mg/d, for the past 2 years. She has not experienced a mood episode while on this regimen, and her car­bamazepine level was 9.2 μg/mL 6 months ago. The only adverse effect she experienced was weight gain of approximately 10 lb. Ms. H takes a calcium supplement, but no other medications.

Ms. H reports to her psychiatrist that, for the past few days, she has been feeling nau­seated, fatigued, and dizzy, but has contin­ued taking her medications as prescribed. Her carbamazepine level is found to be 13.1 μg/mL. Ms. H states she has not started any new medications or supplements; her serum creatinine and liver function test results are within normal limits.

Upon further questioning, Ms. H says that an upper respiratory infection has been “going around her office,” so she increased her vitamin C intake by drinking 2 glasses of grapefruit juice a day (she doesn’t like orange juice). She has heard grapefruit juice can cause problems with some drugs so she is careful not to drink it at the same time she takes her medications. Her psychiatrist recognizes there may be a drug interaction involved, and recommends Ms. H hold her carbamazepine for 1 day and not consume any more grapefruit juice. A few days later, she reports feeling much better during a fol­low-up call and she makes an appointment to have her carbamazepine level rechecked in a we

Although grapefruit products are high in vitamins and low in calories, they can be associated with potentially serious drug interactions. The interaction between grapefruit juice and the calcium channel blocker felodipine was discovered inad­vertently >20 years ago; since that time, possible interactions with >85 medica­tions have been identified.1 Interactions with grapefruit products are complicated because, although most result in increased drug exposure, reduced exposure of the medication also can occur. Additionally, the degree and clinical significance of the interaction varies among individuals and from one drug to another.

Mechanism of action
Most interactions with grapefruit products are thought to result from the inhibition of intestinal cytochrome P450 3A4 (CYP3A4). CYP3A4 is involved in the metabolism of numerous drugs, and is the most abun­dant cytochrome P450 enzyme in the liver and epithelial cells lining the intestine.² Although hepatic CYP3A4 is thought to be minimally affected by grapefruit, inhi­bition of intestinal CYP3A4 can result in an overall increase in bioavailability of medications that are substrates and raise the risk of potential toxicity.³ Grapefruit contains various chemicals collectively known as furanocoumarins, which are largely responsible for inhibition of intes­tinal CYP3A4.⁴ Additionally, Seville oranges and the pomelo (a large, sweet grapefruit-like citrus fruit) also contain furanocoumarins and could have a simi­lar effect, warranting caution with certain medications.⁵

Inhibition of CYP3A4 by furanocouma­rins cannot be reversed, and new enzymes must be synthesized to return to the pre­vious level of function.⁶ Therefore, drug interactions resulting from CYP3A4 inhi­bition can last for as long as 72 hours after ingesting grapefruit products.⁷ Separating consumption of grapefruit products and medication administration will not help manage this interaction.

Grapefruit products also could affect drug disposition through effects on vari­ous drug transporters. Decreased systemic exposure to certain medications could occur through grapefruit’s inhibition of organic anion-transporting polypeptides (OATPs). OATPs form a family of drug uptake transporters found in the intestine, liver, kidney, and brain.8 For drugs that are substrates of OATPs, grapefruit’s inhibition of this transporter can result in decreased absorption and a resulting decrease in effi­cacy. Flavanoids in grapefruit, such as nar­ingin, inhibit OATPs, which is competitive in nature.9 Unlike the irreversible inhibi­tion of CYP3A4 by furanocoumarins, flava­noids effects on OATPs have been shown to decrease within 4 hours.¹⁰

No psychotropic medications have been identified as being susceptible to this interaction, but for those medications affected—including fexofenadine and levothyroxine—separating consumption of grapefruit and medication administra­tion by 4 hours could avoid this interac­tion.¹¹ Additional data indicate that orange juice and apple juice could have similar effects on OATPs.¹²

Perhaps the most well-known drug transporter, P-glycoprotein is part of the multidrug-resistant subfamily of trans­porters. It is located throughout the body, including in the intestine, kidneys, liver, and blood-brain barrier. P-glycoprotein acts as an export pump to decrease the cellular concentration of many different drug substrates, and many agents can alter P-glycoprotein’s expression or function.

Small changes in P-glycoprotein’s activ­ity can result in substantial changes in the disposition of substrates, which can include certain antineoplastics and antiretrovi­rals. Most reports have found grapefruit juice inhibits P-glycoprotein-mediated efflux; however, there also are reports of transporter activation.⁶ Additionally, P-glycoprotein and CYP3A4 share many substrates, so it can be difficult to iso­late the contribution of P-glycoprotein to grapefruit−drug interactions.¹³ The effect of grapefruit on P-glycoprotein activity has been difficult to fully elucidate; more studies are needed.

Grapefruit consumption and its effect
Drug interactions can occur by consum­ing commercially produced grapefruit juice and juice from concentrate, as well as freshly squeezed juice and grapefruit seg­ments.¹⁴ CYP3A4-inhibiting furanocouma­rins also have been isolated in grapefruit peel; it is not known, however, whether items made from peel (marmalade, can­died peel) contain concentrations high enough to pose a risk of a drug interac­tion.¹⁴ Contributing to the unpredictabil­ity of grapefruit-drug interactions, the amount or concentration of furanocou­marins can vary among grapefruit prod­ucts and brands.¹⁵ This variability can be influenced by the variety or maturity of the fruit and the fruit’s exposure to envi­ronmental stress.⁴

The frequency of consuming a grape­fruit product can influence the degree of a drug interaction. In general, consum­ing one 8-oz glass of grapefruit juice or the segments from a whole grapefruit is enough to alter a susceptible drug’s phar­macokinetics.¹⁴ Regular grapefruit product consumption, however, can result in an overall greater effect.^16,17

Lilja et al¹⁶ conducted a randomized, 4-phase, crossover study to look at the effect of grapefruit juice dose on kinetics of triazolam. Grapefruit juice was found to increase the mean area under the concentration-time curve (AUC) of tri­azolam compared with water, but no dif­ference was found between single glasses of normal-strength and double-strength grapefruit juice. However, repeated consumption of double-strength grape­fruit juice (200 mL, 3 times/d for 3 days) increased triazolam’s mean AUC by 143%, compared with an increase of 49% with just a single 200-mL glass of double-strength juice.¹⁶ Recurrent consumption of grapefruit juice (8 oz, 3 times/d for 6 days) also was found to increase the kinetics of the antihypertensive felodipine more than a single glass of grapefruit juice.¹⁷

Clinical consequences of an interaction between a drug and grapefruit can be difficult to predict. Drug concentration changes caused by a grapefruit interac­tion could vary based on interindividual differences. The amount and activity of intestinal CYP3A4 can vary from per­son to person, and can be influenced by genetic polymorphisms in addition to race, age, and environmental variables.¹⁸ Interindividual sensitivity to a change in a drug’s concentration also will differ, and patient-specific factors, such as concomi­tant drugs or diseases, could influence the likelihood of harm.

Interactions with grapefruit products are not necessarily a “class effect,” and specific drugs within a therapeutic cat­egory can be affected (although others might not). Several drug-specific charac­teristics can help gauge the risk of a clini­cally relevant interaction with grapefruit, including:  
   • metabolism through CYP3A4  
   • low bioavailability  
   • oral administration  
   • a narrow therapeutic index.¹

For drugs with low bioavailability because of first-pass metabolism, grape­fruit’s inhibition of intestinal CYP3A4 can result in a greater relative increase in plasma concentrations compared with a drug with high bioavailability.¹⁹

For example, an increase in bioavailabil­ity from 5% to 10% will result in a much larger increase in AUC and overall clinical exposure compared with an increase from 85% to 90% even though both represent an absolute increase of 5%. Although a drug does not have to have low oral bioavail­ability for an interaction to occur, lower bioavailability means that a drug has a higher likelihood of causing a significant interaction because of altered pharmaco­kinetics. Of note, injectable medications will not interact with grapefruit because metabolism through intestinal CYP3A4 is bypassed and grapefruit does not signifi­cantly inhibit hepatic CYP3A4.

Although grapefruit products could alter the pharmacokinetics of susceptible drugs, those changes might not be associated with adverse effects. Therefore, a factor to con­sider in evaluating a potential interaction with grapefruit is the drug’s therapeutic index and its risk of serious adverse effects. Drugs with a narrow therapeutic index are of particular concern because a significant increase in therapeutic or adverse effects could result from a relatively small increase in the drug’s concentration.⁷

Which medications are affected?
Among medications identified as interact­ing with grapefruit, some cardiovascular agents and several of the HMG-CoA reduc­tase inhibitors (statins) have garnered the most attention. However, grapefruit also can affect the metabolism of several psy­chotropic medications through inhibition of intestinal CYP3A4 (Table).^16,20-35 Prescribing information for some drugs warns against consuming grapefruit while using the medication. Among CNS agents, buspirone, carbamazepine, lurasidone, pimozide, triazolam, and oral midazolam all have such warnings in their product labeling.

Buspirone currently is not recommended with “large quantities of grapefruit juice.”²⁰ A randomized, 2-phase crossover study looking at the effects of grapefruit juice on buspirone’s pharmacokinetics found that double-strength grapefruit juice (200 mL, administered 3 times/d for 3 days) resulted in a 9.2-fold increase in mean AUC and a 4.3-fold increase in mean Cmax after a single 10-mg buspirone dose.22 Highlighting the wide interindivid­ual variability seen with drug-grapefruit interactions, the increase found in buspi­rone’s AUC ranged from 3-fold to 20-fold among study participants.²²

Carbamazepine product labeling lists grapefruit juice as a CYP3A4 inhibi­tor that is expected to or has been found to increase plasma levels of the drug.²⁰ Carbamazepine’s bioavailability is influ­enced by intestinal CYP3A4 activity; in a randomized, 2-phase crossover study of 10 patients with epilepsy, grapefruit juice was found to increase AUC of carbamaze­pine by 41% and Cmax by 40%.^23,36

Lurasidone and pimozide, although not specifically studied, have product labels that recommend avoiding grapefruit juice because it could inhibit metabolism of these agents by CYP3A4.²⁰ Of particular concern is the potential for elevated levels of pimozide to increase the risk of adverse cardiovascular effects including QT inter­val prolongation.¹⁹

Midazolam. Although grapefruit juice does not affect the disposition of IV mid­azolam, pretreatment with grapefruit juice was found to increase the AUC and Cmax of oral midazolam by 52% and 56%, respectively.³⁰
 

Other considerations in drug-grapefruit interactions
Cautionary statements about a possible interaction with grapefruit juice for many other psychotropics can be found in com­monly used drug information references or online sources. If you are concerned about a possible interaction and avoiding grape­fruit products is not feasible, consider a different medication in the same class.

However, you also should consider the level of evidence supporting any pur­ported interaction. Several psychotropic agents do have studies or case reports sup­porting an interaction with grapefruit, but cautionary statements could be based on theoretical concerns because of a medica­tion’s bioavailability, metabolic pathway, and concern for increased adverse events related to higher drug concentrations. Adding to the confusion, cautionary state­ments can be found about medications, such as clozapine, that have not been shown to have an interaction with grape­fruit juice when studied.

With many of the drugs that have a reported or theoretical interaction with grapefruit, data are inconsistent as to whether the resulting interaction will be clinically relevant. A number of variables relating to the individual patient, grape­fruit product, or particular drug can play a role in the significance of an interaction. Additionally, effects on drug disposition can last for a few days after consuming a grapefruit product.

Keep alert to situations of increased risk
Recall that the case patient, Ms. H, pre­sented with an elevated carbamazepine level and suffered resulting adverse effects because of an interaction between the drug and grapefruit juice. Although Ms. H was careful to separate intake of grapefruit juice from carbamazepine administra­tion, grapefruit’s inhibition of intestinal CYP3A4 still was present, leading to the interaction.

It is important for health care profes­sionals to recognize this potential risk and to advise patients regarding possible inter­actions between medications and grape­fruit products.

Related Resources
• U.S. Food and Drug Administration. Grapefruit juice and medicine may not mix. http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm292276.htm.
• Hanley MJ, Cancalon P, Widmer WW, et al. The ef­fect of grapefruit juice on drug disposition. Expert Opin Drug Metab Toxicol. 2011;7(3):267-286.
• Andrade C. Fruit juice, organic anion transporting polypeptides, and drug interactions in psychiatry. J Clin Psychiatry. 2014;75(11):e1323-e1325.
 

Drug Brand Names
Alprazolam • Xanax                                     Lurasidone • Latuda
Buspirone • BuSpar                                     Midazolam • Versed
Carbamazepine • Tegretol                            Methadone • Dolophine
Clomipramine • Anafranil                             Nefazodone • Serzone
Clozapine • Clozaril                                     Olanzapine • Zyprexa
Diazepam • Valium                                     Pimozide • Orap
Felodipine • Plendil                                     Quetiapine • Seroquel
Fexofenadine • Allegra                                Sertraline • Zoloft
Fluoxetine • Prozac                                    Trazodone • Desyrel
Fluvoxamine • Luvox                                  Triazolam • Halcion
Haloperidol • Haldol                                   Ziprasidone • Geodon
Levothyroxine • Levoxyl, Synthroid   

Disclosure
The author reports no financial relationships with any company whose products are mentioned in this article or with manufacturers of competing products.

Ms. H, age 42, was given a diagnosis of bipolar disorder 10 years ago and has been taking carbamazepine, 1,200 mg/d, and olanzapine, 10 mg/d, for the past 2 years. She has not experienced a mood episode while on this regimen, and her car­bamazepine level was 9.2 μg/mL 6 months ago. The only adverse effect she experienced was weight gain of approximately 10 lb. Ms. H takes a calcium supplement, but no other medications.

Ms. H reports to her psychiatrist that, for the past few days, she has been feeling nau­seated, fatigued, and dizzy, but has contin­ued taking her medications as prescribed. Her carbamazepine level is found to be 13.1 μg/mL. Ms. H states she has not started any new medications or supplements; her serum creatinine and liver function test results are within normal limits.

Upon further questioning, Ms. H says that an upper respiratory infection has been “going around her office,” so she increased her vitamin C intake by drinking 2 glasses of grapefruit juice a day (she doesn’t like orange juice). She has heard grapefruit juice can cause problems with some drugs so she is careful not to drink it at the same time she takes her medications. Her psychiatrist recognizes there may be a drug interaction involved, and recommends Ms. H hold her carbamazepine for 1 day and not consume any more grapefruit juice. A few days later, she reports feeling much better during a fol­low-up call and she makes an appointment to have her carbamazepine level rechecked in a we

Although grapefruit products are high in vitamins and low in calories, they can be associated with potentially serious drug interactions. The interaction between grapefruit juice and the calcium channel blocker felodipine was discovered inad­vertently >20 years ago; since that time, possible interactions with >85 medica­tions have been identified.1 Interactions with grapefruit products are complicated because, although most result in increased drug exposure, reduced exposure of the medication also can occur. Additionally, the degree and clinical significance of the interaction varies among individuals and from one drug to another.

Mechanism of action
Most interactions with grapefruit products are thought to result from the inhibition of intestinal cytochrome P450 3A4 (CYP3A4). CYP3A4 is involved in the metabolism of numerous drugs, and is the most abun­dant cytochrome P450 enzyme in the liver and epithelial cells lining the intestine.² Although hepatic CYP3A4 is thought to be minimally affected by grapefruit, inhi­bition of intestinal CYP3A4 can result in an overall increase in bioavailability of medications that are substrates and raise the risk of potential toxicity.³ Grapefruit contains various chemicals collectively known as furanocoumarins, which are largely responsible for inhibition of intes­tinal CYP3A4.⁴ Additionally, Seville oranges and the pomelo (a large, sweet grapefruit-like citrus fruit) also contain furanocoumarins and could have a simi­lar effect, warranting caution with certain medications.⁵

Inhibition of CYP3A4 by furanocouma­rins cannot be reversed, and new enzymes must be synthesized to return to the pre­vious level of function.⁶ Therefore, drug interactions resulting from CYP3A4 inhi­bition can last for as long as 72 hours after ingesting grapefruit products.⁷ Separating consumption of grapefruit products and medication administration will not help manage this interaction.

Grapefruit products also could affect drug disposition through effects on vari­ous drug transporters. Decreased systemic exposure to certain medications could occur through grapefruit’s inhibition of organic anion-transporting polypeptides (OATPs). OATPs form a family of drug uptake transporters found in the intestine, liver, kidney, and brain.8 For drugs that are substrates of OATPs, grapefruit’s inhibition of this transporter can result in decreased absorption and a resulting decrease in effi­cacy. Flavanoids in grapefruit, such as nar­ingin, inhibit OATPs, which is competitive in nature.9 Unlike the irreversible inhibi­tion of CYP3A4 by furanocoumarins, flava­noids effects on OATPs have been shown to decrease within 4 hours.¹⁰

No psychotropic medications have been identified as being susceptible to this interaction, but for those medications affected—including fexofenadine and levothyroxine—separating consumption of grapefruit and medication administra­tion by 4 hours could avoid this interac­tion.¹¹ Additional data indicate that orange juice and apple juice could have similar effects on OATPs.¹²

Perhaps the most well-known drug transporter, P-glycoprotein is part of the multidrug-resistant subfamily of trans­porters. It is located throughout the body, including in the intestine, kidneys, liver, and blood-brain barrier. P-glycoprotein acts as an export pump to decrease the cellular concentration of many different drug substrates, and many agents can alter P-glycoprotein’s expression or function.

Small changes in P-glycoprotein’s activ­ity can result in substantial changes in the disposition of substrates, which can include certain antineoplastics and antiretrovi­rals. Most reports have found grapefruit juice inhibits P-glycoprotein-mediated efflux; however, there also are reports of transporter activation.⁶ Additionally, P-glycoprotein and CYP3A4 share many substrates, so it can be difficult to iso­late the contribution of P-glycoprotein to grapefruit−drug interactions.¹³ The effect of grapefruit on P-glycoprotein activity has been difficult to fully elucidate; more studies are needed.

Grapefruit consumption and its effect
Drug interactions can occur by consum­ing commercially produced grapefruit juice and juice from concentrate, as well as freshly squeezed juice and grapefruit seg­ments.¹⁴ CYP3A4-inhibiting furanocouma­rins also have been isolated in grapefruit peel; it is not known, however, whether items made from peel (marmalade, can­died peel) contain concentrations high enough to pose a risk of a drug interac­tion.¹⁴ Contributing to the unpredictabil­ity of grapefruit-drug interactions, the amount or concentration of furanocou­marins can vary among grapefruit prod­ucts and brands.¹⁵ This variability can be influenced by the variety or maturity of the fruit and the fruit’s exposure to envi­ronmental stress.⁴

The frequency of consuming a grape­fruit product can influence the degree of a drug interaction. In general, consum­ing one 8-oz glass of grapefruit juice or the segments from a whole grapefruit is enough to alter a susceptible drug’s phar­macokinetics.¹⁴ Regular grapefruit product consumption, however, can result in an overall greater effect.^16,17

Lilja et al¹⁶ conducted a randomized, 4-phase, crossover study to look at the effect of grapefruit juice dose on kinetics of triazolam. Grapefruit juice was found to increase the mean area under the concentration-time curve (AUC) of tri­azolam compared with water, but no dif­ference was found between single glasses of normal-strength and double-strength grapefruit juice. However, repeated consumption of double-strength grape­fruit juice (200 mL, 3 times/d for 3 days) increased triazolam’s mean AUC by 143%, compared with an increase of 49% with just a single 200-mL glass of double-strength juice.¹⁶ Recurrent consumption of grapefruit juice (8 oz, 3 times/d for 6 days) also was found to increase the kinetics of the antihypertensive felodipine more than a single glass of grapefruit juice.¹⁷

Clinical consequences of an interaction between a drug and grapefruit can be difficult to predict. Drug concentration changes caused by a grapefruit interac­tion could vary based on interindividual differences. The amount and activity of intestinal CYP3A4 can vary from per­son to person, and can be influenced by genetic polymorphisms in addition to race, age, and environmental variables.¹⁸ Interindividual sensitivity to a change in a drug’s concentration also will differ, and patient-specific factors, such as concomi­tant drugs or diseases, could influence the likelihood of harm.

Interactions with grapefruit products are not necessarily a “class effect,” and specific drugs within a therapeutic cat­egory can be affected (although others might not). Several drug-specific charac­teristics can help gauge the risk of a clini­cally relevant interaction with grapefruit, including:  
   • metabolism through CYP3A4  
   • low bioavailability  
   • oral administration  
   • a narrow therapeutic index.¹

For drugs with low bioavailability because of first-pass metabolism, grape­fruit’s inhibition of intestinal CYP3A4 can result in a greater relative increase in plasma concentrations compared with a drug with high bioavailability.¹⁹

For example, an increase in bioavailabil­ity from 5% to 10% will result in a much larger increase in AUC and overall clinical exposure compared with an increase from 85% to 90% even though both represent an absolute increase of 5%. Although a drug does not have to have low oral bioavail­ability for an interaction to occur, lower bioavailability means that a drug has a higher likelihood of causing a significant interaction because of altered pharmaco­kinetics. Of note, injectable medications will not interact with grapefruit because metabolism through intestinal CYP3A4 is bypassed and grapefruit does not signifi­cantly inhibit hepatic CYP3A4.

Although grapefruit products could alter the pharmacokinetics of susceptible drugs, those changes might not be associated with adverse effects. Therefore, a factor to con­sider in evaluating a potential interaction with grapefruit is the drug’s therapeutic index and its risk of serious adverse effects. Drugs with a narrow therapeutic index are of particular concern because a significant increase in therapeutic or adverse effects could result from a relatively small increase in the drug’s concentration.⁷

Which medications are affected?
Among medications identified as interact­ing with grapefruit, some cardiovascular agents and several of the HMG-CoA reduc­tase inhibitors (statins) have garnered the most attention. However, grapefruit also can affect the metabolism of several psy­chotropic medications through inhibition of intestinal CYP3A4 (Table).^16,20-35 Prescribing information for some drugs warns against consuming grapefruit while using the medication. Among CNS agents, buspirone, carbamazepine, lurasidone, pimozide, triazolam, and oral midazolam all have such warnings in their product labeling.

Buspirone currently is not recommended with “large quantities of grapefruit juice.”²⁰ A randomized, 2-phase crossover study looking at the effects of grapefruit juice on buspirone’s pharmacokinetics found that double-strength grapefruit juice (200 mL, administered 3 times/d for 3 days) resulted in a 9.2-fold increase in mean AUC and a 4.3-fold increase in mean Cmax after a single 10-mg buspirone dose.22 Highlighting the wide interindivid­ual variability seen with drug-grapefruit interactions, the increase found in buspi­rone’s AUC ranged from 3-fold to 20-fold among study participants.²²

Carbamazepine product labeling lists grapefruit juice as a CYP3A4 inhibi­tor that is expected to or has been found to increase plasma levels of the drug.²⁰ Carbamazepine’s bioavailability is influ­enced by intestinal CYP3A4 activity; in a randomized, 2-phase crossover study of 10 patients with epilepsy, grapefruit juice was found to increase AUC of carbamaze­pine by 41% and Cmax by 40%.^23,36

Lurasidone and pimozide, although not specifically studied, have product labels that recommend avoiding grapefruit juice because it could inhibit metabolism of these agents by CYP3A4.²⁰ Of particular concern is the potential for elevated levels of pimozide to increase the risk of adverse cardiovascular effects including QT inter­val prolongation.¹⁹

Midazolam. Although grapefruit juice does not affect the disposition of IV mid­azolam, pretreatment with grapefruit juice was found to increase the AUC and Cmax of oral midazolam by 52% and 56%, respectively.³⁰
 

Other considerations in drug-grapefruit interactions
Cautionary statements about a possible interaction with grapefruit juice for many other psychotropics can be found in com­monly used drug information references or online sources. If you are concerned about a possible interaction and avoiding grape­fruit products is not feasible, consider a different medication in the same class.

However, you also should consider the level of evidence supporting any pur­ported interaction. Several psychotropic agents do have studies or case reports sup­porting an interaction with grapefruit, but cautionary statements could be based on theoretical concerns because of a medica­tion’s bioavailability, metabolic pathway, and concern for increased adverse events related to higher drug concentrations. Adding to the confusion, cautionary state­ments can be found about medications, such as clozapine, that have not been shown to have an interaction with grape­fruit juice when studied.

With many of the drugs that have a reported or theoretical interaction with grapefruit, data are inconsistent as to whether the resulting interaction will be clinically relevant. A number of variables relating to the individual patient, grape­fruit product, or particular drug can play a role in the significance of an interaction. Additionally, effects on drug disposition can last for a few days after consuming a grapefruit product.

Keep alert to situations of increased risk
Recall that the case patient, Ms. H, pre­sented with an elevated carbamazepine level and suffered resulting adverse effects because of an interaction between the drug and grapefruit juice. Although Ms. H was careful to separate intake of grapefruit juice from carbamazepine administra­tion, grapefruit’s inhibition of intestinal CYP3A4 still was present, leading to the interaction.

It is important for health care profes­sionals to recognize this potential risk and to advise patients regarding possible inter­actions between medications and grape­fruit products.

Related Resources
• U.S. Food and Drug Administration. Grapefruit juice and medicine may not mix. http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm292276.htm.
• Hanley MJ, Cancalon P, Widmer WW, et al. The ef­fect of grapefruit juice on drug disposition. Expert Opin Drug Metab Toxicol. 2011;7(3):267-286.
• Andrade C. Fruit juice, organic anion transporting polypeptides, and drug interactions in psychiatry. J Clin Psychiatry. 2014;75(11):e1323-e1325.
 

Drug Brand Names
Alprazolam • Xanax                                     Lurasidone • Latuda
Buspirone • BuSpar                                     Midazolam • Versed
Carbamazepine • Tegretol                            Methadone • Dolophine
Clomipramine • Anafranil                             Nefazodone • Serzone
Clozapine • Clozaril                                     Olanzapine • Zyprexa
Diazepam • Valium                                     Pimozide • Orap
Felodipine • Plendil                                     Quetiapine • Seroquel
Fexofenadine • Allegra                                Sertraline • Zoloft
Fluoxetine • Prozac                                    Trazodone • Desyrel
Fluvoxamine • Luvox                                  Triazolam • Halcion
Haloperidol • Haldol                                   Ziprasidone • Geodon
Levothyroxine • Levoxyl, Synthroid   

Disclosure
The author reports no financial relationships with any company whose products are mentioned in this article or with manufacturers of competing products.