Drinking fruit juices may not be as healthy an option as thought - they could reduce the effectiveness of some medicines, it is being claimed.
Research presented at a US conference suggested a chemical in grapefruit juice could stop anti-allergy drugs being absorbed properly.
A University of Western Ontario team said oranges, and possibly apples, had similar ingredients.
Grapefruit juice is already known to interfere with blood pressure drugs.
Some medications carry a warning that taking them alongside grapefruit juice could cause an overdose.
However, the latest finding, presented at the American Chemical Society conference in Philadelphia, points to a different problem with researchers saying it was "the tip of the iceberg".
In this case, he found that the grapefruit juice had the reverse effect on fexofenadine, an antihistamine drug, making it less potent rather than more potent.
Volunteers took the drug with either a single glass of grapefruit juice, or just water.
When it was taken with juice, only half the drug was absorbed, potentially reducing its effectiveness.
Researchers believe that an active ingredient of the juice, naringin, appears to block a mechanism which moves drug molecules out of the small intestine into the bloodstream.
Study author Dr David Bailey said that orange and apple juices appeared to contain naringin-like substances which might have a similar effect.
"Recently, we discovered that grapefruit and these other fruit juices substantially decrease the oral absorption of certain drugs undergoing intestinal uptake transport.
"The concern is loss of benefit of medications essential for the treatment of serious medical conditions."
So far, the three types of juice have been found to affect etoposide, a chemotherapy drug, some beta-blocker drugs used to treat high blood pressure, and cyclosporine, taken by transplant patients to prevent rejection of their new organs.
However, Dr Bailey said: "This is just the tip of the iceberg - I'm sure we'll find more and more drugs that are affected this way."
Colette McCready, from the National Pharmacy Association, said: "The effect of grapefruit juice on some medicines is well established and where this applies it is clearly detailed in Patient Information Leaflets.
"Pharmacists will usually draw this matter to patients' attention when dispensing their medicines. This new research showing that apple and orange juice may enhance or reduce the effects of some medicines is interesting but it is only one study.
"Usually further research is needed to establish that these interactions are significant."
Professor James Ritter, a clinical pharmacologist at King's College London, said: "The observation is very interesting. It will need more work to establish how important such interactions are in clinical practice and for what drugs and juices."
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