Grapefruit, orange and apple juices can harm the body's ability to absorb certain medications and make the drugs less effective, said a Canadian study released last week in the United States.

The research showed that these juices can decrease the effectiveness of certain drugs used to treat heart disease, cancer, organ-transplant rejection and infection, "potentially wiping out their beneficial effects," it said.

David Bailey, a professor of clinical pharmacology with the University of Western Ontario and leader of the study, was the first researcher to identify grapefruit juice's potential to increase the absorption of certain drugs two decades ago, possibly turning some doses toxic.

The new findings came as part of his continuing research on the subject, and were presented at the 236th annual meeting of the American Chemical Society on Philadelphia, Pennsylvania.

"Recently, we discovered that grapefruit and these other fruit juices substantially decrease the oral absorption of certain drugs undergoing intestinal uptake transport," AFP quoted Bailey as saying.

"The concern is loss of benefit of medications essential for the treatment of serious medical conditions."

Healthy volunteers took fexofenadine, an antihistamine used to fight allergies, along with either a glass of grapefruit juice, a glass of water with naringin (which gives the bitter taste to grapefruit juice), or plain water.

Those who drank the grapefruit juice absorbed only half the amount of fexofenadine, compared to those who drank plain water.

Researchers said the water with naringin served to block "a key drug uptake transporter, called OATP1A2, involved in shuttling drugs from the small intestine to the bloodstream."

Among the drugs affected by consumption of grapefruit, orange and apple juices are: etoposide, an anticancer agent; beta blockers (atenolol, celiprolol, talinolol) used to treat high blood pressure and prevent heart attacks; and certain antibiotics (ciprofloxacin, levofloxacin, itraconazole). More drugs were expected to be added to the list as the research continued.

Bailey said patients should consult with a doctor about taking medications with juice, and stick to plain water when taking most medications.