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Actually, grapefruit juice more than grapefruit itself seems to be the problem – something it contains interferes with the absorption and metabolism of a number of drugs (although there is increasing concern that eating grapefruit might do the same). The affected drugs include calcium channel blockers used to treat high blood pressure, non-sedating antihistamines such as Hismanal (astemizole), certain tranquilizers including Valium (diazepam), Xanax (alprazolam), Halcion (triazolam), and others, a number of cholesterol-lowering drugs, including Lipitor (atorvastatin), Mevacor (lovastatin) and other statins, and the immunosuppressive drugs used to prevent rejection of transplanted tissues organs and the antiviral agents used to treat HIV/AIDS.

If you take any of these drugs with grapefruit juice, you may feel a difference. With calcium channel blockers you might notice flushing, headache, an increased heart rate, or blood pressure which is lower than intended. With the tranquilizers, you may notice increased sedation. More dangerous side effects including kidney toxicity; increased susceptibility to infections could develop among those taking immunosuppressive drugs. Grapefruit juice can even enhance your response to caffeine, resulting in nervousness and over-stimulation.

Orange juice doesn't appear to have these effects and neither do any other types of juice made from citrus fruits.

The compounds believed responsible for the effect of grapefruit juice are furanocoumarins, which interfere with an intestinal enzyme, CYP3A. Normally, CYP3A partially metabolizes and eliminates drugs as they are absorbed, but the furanocoumarins in grapefruit juice inhibit this process, allowing more of the drugs to enter into the bloodstream. Researchers say that it should be possible to remove furanocoumarins from grapefruit juice to produce a product that patients on affected medications can enjoy.

Andrew Weil, M.D.