Drinking grapefruit, apple or orange juice may wipe out part of the benefits of taking medications for some life-threatening conditions, Canadian researchers say.

While grapefruit juice has been known for years to increase the absorption of certain drugs, potentially leading to toxic overdoses during what is known as the so-called "grapefruit juice effect," pharmacology researchers have now found that the opposite may also happen.

Pharmacology Prof. David Bailey of the University of Western Ontario found healthy volunteers who took fexofenadine, an antihistamine, with grapefruit juice, absorbed only half the drug, compared to those who downed the pill with water.

The study also suggested that naringin, a substance in grapefruit juice that gives the juice its bitter taste, may block a key transporter that is needed to shuttle drugs from the small intestine to the bloodstream.

Blocking the transporter reduced drug absorption and neutralized its potential benefits, the researchers said.

So far, grapefruit, orange and apple juices have been shown to lower the absorption of medications including:

• Etoposide, an anticancer agent.
• Certain beta blockers (atenolol, celiprolol, talinolol) used to treat high blood pressure and prevent heart attacks.
• Cyclosporine, a drug taken to prevent rejection of transplanted organs.
• Certain antibiotics (ciprofloxacin, levofloxacin, itraconazole).

"These are three serious medical conditions for which the drugs simply have to work properly," Bailey said.

Bailey called the findings "just the tip of the iceberg," saying he believes more drugs will be added to the list as doctors become more aware of the drug-lowering interaction.

He advised patients to check with their doctor or pharmacist before taking any medication with juice, and recommended taking most drugs with only water.

The findings were presented Tuesday at the American Chemical Society's national meeting in Philadelphia. The research was funded by grants from the Canadian Institutes of Health Research and the United States Public Health Service.

With files from the Canadian Press