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Downing a morning glass of grapefruit or other juice may be a great way to get vitamin C. But for those taking certain medications, the common breakfast beverage could also do serious harm.

A Canadian study shows that drinking the juices can dramatically decrease the body's ability to absorb certain drugs — including some taken for life-threatening conditions like cancer and heart disease.

Researchers at the University of Western Ontario in London found that subjects who took a particular antihistamine with grapefruit juice absorbed only half the drug compared to those who took the pill with water.

Other drugs that are absorbed in a similar way as the antihistamine would likely be affected in the same way, the researchers say.

“The concern is basically for a loss of effect for drugs, and particularly those ... used in serious medical conditions,” principal researcher David Bailey said Tuesday from Philadelphia, where he presented the study's findings at the national meeting of the American Chemical Society.

Mr. Bailey, a professor of clinical pharmacology, said an active ingredient in grapefruit juice appears to block a transporter protein that allows absorption of certain drugs from the small intestine into the bloodstream. Orange and apple juices contain similar ingredients.

The drugs include an anti-cancer agent, some beta blockers that can prevent a second heart attack and a medication to prevent rejection of transplanted organs, he said.

“These are three serious medical conditions for which the drugs simply have to work properly.”

In 1990, Mr. Bailey reported on the first known food-drug interaction, when his team discovered that grapefruit juice boosted the potency of the high blood pressure drug felodipine, leading to toxic levels of drug in the blood.

Since then, other researchers have identified about 50 medications that are subject to the so-called “grapefruit juice effect.” Some prescription drugs now carry warning labels against consuming the juice or fresh grapefruit while taking the medications to avoid overdose.

The new study shows that grapefruit, orange and apple juices — and also the whole fruits — can have the opposite effect, by limiting absorption.

To date, the fruit juices have been shown to lower the absorption of the anti-cancer agent etoposide; the beta blockers atenolol, celiprolol and talinolol; the anti-rejection drug cyclosporine; and the antibiotics ciprofloxacin, levofloxacin and itraconazole.

“This is just the tip of the iceberg,” said Bailey. “I'm sure we'll find more and more drugs that are affected this way.”

Commenting on the research, Dr. David Juurlink of Sunnybrook Health Sciences Centre in Toronto said there are probably dozens of transporter proteins that pump drugs in and out of tissues all over the body that could be affected by what we ingest.

“And we have only a very rudimentary understanding of what these pumps do and what drugs they affect and what other drugs or foods might modulate these pumps,” said Dr. Juurlink, Sunnybrook's head of clinical pharmacology and toxicology.

“There's a lot of uncertainty in terms of what the implications are for the average person.”

Mr. Bailey said patients should check with their doctor or pharmacist before taking a drug with grapefruit or other fruits and juices. As a general rule, he suggests avoiding consumption for about four hours before and after taking a pill.

“The best thing to do in general to get the most consistent effect of your drug ... is to take it with a glass of water on an empty stomach.”

Dr. Juurlink said that while the four-hour window is reasonable advice, there's too little known about the complexity of food-drug interactions to come up with a hard-and-fast rule for all patients and all medications.

“It's hard to formulate a generic recommendation that fits everybody, other than be aware of the fact that food — and grapefruit in particular — can influence how your body handles drugs.”