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Grapefruit juice and medications

AUGUSTA, GEORGIA. Grapefruit juice is known to enhance the absorption (bioavailability) of the calcium channel blocker felodipine (Plendil, Renedil). Initial studies showed that taking 15 mg of felodipine with 250 ml of double- strength grapefruit juice enhanced the absorption of the drug by 284 per cent as compared to when it was taken with water; later it was found that 200 ml of regular grapefruit juice tripled the bioavailability of felodipine. Other citrus juices (orange and lemon) do not have the medication-enhancing effects of grapefruit juice. More recent studies have shown that grapefruit juice also enhances the absorption of nifedipine (Adalat, Procardia), verapamil, terfenadine (Seldane), the hormone ethinyl estradiol (Estinyl), medazolam, cyclosporine, and the antiviral agent saquinavir. Grapefruit juice contains flavonoids (quercetin, kaempferol, naringenin and 6'7' dihydroxybergamotten) and it is believed that one or more of these compounds are responsible for the drug enhancement effect. However, experiments using quercetin and naringenin on their own have not been successful. A recent experiment concluded that grapefruit juice prevents the normal intestinal metabolism of drugs catalyzed by the cytochrome P450 enzymes by inactivating a specific enzyme, CYP3A4. The experiment showed an average drop of CYP3A4 concentration in the small intestine of 62 per cent with consumption of grapefruit juice. It also concluded that the effect of grapefruit juice increases with regular intake. Researchers speculate that it may be possible to add the effective components of grapefruit juice to susceptible medications to increase their reliability and lower their cost. Some of the flavonoids found in grapefruit juice are also found in grapes, grape juice, and strawberries. NOTE: Patients taking medications should be aware of these potential drug-food interactions and consult with their physician to adjust their dosages as necessary.
Feldman, Elaine B. How grapefruit juice potentiates drug bioavailability. Nutrition Reviews, Vol. 55, November 1997, pp. 398-400

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