Fruit juices including grapefruit, orange and apple can dramatically reduce the effectiveness of heart and cancer drugs, it has been claimed.
Potentially the medicines could be rendered useless if they are taken at the same time as drinking juice, Canadian researchers say.
The evidence emerged when grapefruit was found to shut certain drugs out of the body.
Other fruit juices, notably orange and apple, are thought to have the same effect.
Patients consuming fruit or juice run the risk of wiping out the benefits of their medicines — among them vital treatments for heart disease, cancer, organ transplant rejection and infection, the scientists warned.
For 20 years it has been known that grapefruit can boost the potency of some drugs, increasing the risk of an overdose.
The new research, presented at the national meeting of the American Chemical Society in Philadelphia, shows that other drugs may be affected in the opposite way by fruit juices.
In tests, healthy volunteers took fexofenadine — an antihistamine used to fight allergies.
They swallowed the drug with either a single glass of grapefruit juice, water containing naringin — the chemical responsible for the fruit’s bitter taste — or plain water.
When the medicine was taken with grapefruit juice, only half as much of the drug was absorbed as with water.
Professor David Bailey, from the University of Western Ontario, said losing half of a drug dose could be critical.
"The concern is loss of benefit of medications essential for the treatment of serious medical conditions," he said.
Prof Bailey urged patients to consult their doctor or pharmacist before taking any medications with grapefruit juice or other fruits and juices. In general, most medicines should only be taken with water, he said.