How important is it for patients to avoid grapefruit/grapefruit juice when taking medication, particularly for cardiovascular disease? What amount of grapefruit could be consumed without causing concern?—EVERETT H. KARON, MD, St. Paul, Minn.

As you know, grapefruit juice is a potent inhibitor of CYP3A, an isoenzyme in the liver’s cytochrome P450 system whereby the liver metabolizes drugs through oxidation or reduction to facilitate excretion by the kidneys. Grapefruit juice appears to downregulate intestinal CYP3A, thus increasing the bioavailability of certain drugs, including the dihydropyridine calcium channel blockers felodipine and nifedipine. In one study, 300 mL of grapefruit juice was shown to increase plasma concentrations of midazolam by nearly a factor of two, although these effects were largely washed out within three days (Clin Pharmacol Ther. 2003;74:121-129). If your patient is taking these drugs, you are right to urge caution.—Daniel G. Tobin, MD (146-13)