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.
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