Grapefruit juice, already known to increase the toxicity of certain drugs, has now been found to have an opposite but equally harmful interaction with other medications. Orange and apple juices have also been implicated in this latest discovery.

Nearly 20 years ago,  a team led by clinical pharmacologist David G. Bailey, PhD, discovered that grapefruit juice could increase the levels of the antihypertensive felodipine (Plendil) in the body to dangerous concentrations. Nearly 50 additional medications vulnerable to this effect have since been identified.

Recently, Dr. Bailey, professor of clinical pharmacology at University of Western Ontario, in London, Ont., and his team found that grapefruit, orange, and apple juices can significantly reduce the intestinal absorption of some medications, rendering them largely ineffective. “The concern is a loss of benefit of medications essential for the treatment of serious medical conditions,” says Dr. Bailey, who presented his paper at the  meeting of the American Chemical Society in Philadelphia.

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In healthy volunteers taking fexofenadine (Allegra) with grapefruit juice rather than with water, only half the drug was absorbed. The researchers discovered that in the presence of some drugs, naringin (the active ingredient in grapefruit juice) appears to block a drug-uptake transporter that moves from the small intestine to the bloodstream. As a result, the medication is not fully absorbed and its benefits are reduced.

Orange and apple juices appear to contain naringinlike substances that lower the effectiveness of some beta blockers, e.g., atenolol (Tenormin); the antibiotics ciprofloxacin (e.g., Cipro), levofloxacin (Levaquin), and itraconazole (Sporanox); and other drugs.