The Grapefruit Juice Effect

If you swallow prescription pills with grapefruit juice, there is something very important that you should know:

Grapefruit juice can intensify the effects of some drugs because it can block the first phase of liver detoxification. This grapefruit juice effect is attributed to a constituent of grapefruit juice called naringenin. Naringenin inhibits an enzyme called cytochrome P450, which is found not only in the liver but also in the wall of the intestines. Naringenin gives grapefruit juice its characteristic color and flavor.

Studies by Dr. David Bailey, Dr. Malcolm Arnold and Dr. J. David Spence indicate that the consumption of grapefruit juice may have a significant effect on raising the blood levels of certain drugs like the antihistamine terfenadine (Seldane®). Seldane® has been implicated in at least 125 deaths. While still on the market, it now requires a prescription. Fatal heart arrhythmias might develop in given individuals as a direct result of the sustained high blood levels of this drug. The grapefruit juice effect is variable among individuals with drug levels being boosted anywhere from three to six times.

Besides terfenadine, other drugs affected by grapefruit juice are the calcium channel blocker nifedipine (Adalat®), the anti-rejection drug cyclosporin, prescription estrogens, and the sedative triazolam (Halcion®). Calcium channel blockers are used to control high blood pressure and angina. Nifedipine is one of the controversial calcium channel blocking drugs associated with a greater risk of fatal heart attacks and cancer in several American studies. Blood levels of these drugs can jump as much as six-fold in some individuals after drinking grapefruit juice.

If you must take any of these drugs, the best advice is to avoid grapefruit juice even within 24 hours of taking the tablets or capsules. Use water or orange juice to help swallow the prescriptions since neither will have any significant effect on the blood levels of any of the drugs in question. It is always a good policy to consult a knowledgeable pharmacist about drug-nutrient interactions when taking any prescription or over-the-counter drug.

Dr. Rona

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