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Grapefruit juice may interfere with certain medications. (Warn Patient of Interactions).

BETHESDA, MD. -- The health benefits of drinking grapefruit juice. must be balanced against the risk that it will interfere with certain drugs, Dr. Paul Watkins said at a symposium on drug interactions with herbal products and food.

Compounds called furocoumarins in grapefruit juice increase the bioavailabiity of several drugs by interfering with enzyme activity in the small intestine. The effect is most clinically relevant if a patient is a "high metabolizer" of a drug, said Dr. Watkins of the University of North Carolina at Chapel Hill.

When considering such interactions, keep in mind that certain individuals have naturally low rates of drug absorption. For some people on a drug like lovastatin, grapefruit juice will increase their area under the curve (a measure of drug absorption over time) by 30-fold, "but they were the people who had very low area under the curve to begin with," he said.

Given the large number of available medications, relatively few have been clearly shown to interact with grapefruit juice. Dr. Watkins offered this list, citing changes in drug absorption expressed in terms of the area under the curve:

* Felodipine. This calcium channel blocker was the first drug to show interference by grapefruit juice, with a threefold increase in absorption.

* Lovastatin/Simvastatin. Grapefruit juice causes an average 10-fold increase in absorption of these cholesterol-lowering agents.

* Cyclosporine. Grapefruit juice causes a 1.5-fold increase in absorption of this immunosuppressant.

* Saquinavir. Grapefruit juice causes a twofold increase in absorption of this protease inhibitor approved to treat HIV.

The group of compounds called furocoumarins appears to be the active substance in grapefruit juice that interferes with certain drugs. This group includes bergamottin, dihydroxybergamottin (DHB), and a variety of related dimers. DHB appears to be the major active compound, Dr. Watkins said at the meeting, also sponsored by the American Society for Clinical Pharmacology and Therapeutics.
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Author:Splete, Heidi
Publication:Internal Medicine News
Date:Nov 15, 2002
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