Fruit juices, such as grapefruit, orange and apple, can seriously reduce the effectiveness of heart and cancer drugs, scientists have claimed.

Canadian researchers said the medicines could be rendered useless if they are taken at the same time as drinking juice.

The evidence emerged from a study in which grapefruit juice 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 to be healthy 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 juice can boost the potency of some drugs, increasing the risk of an overdose.

A number of prescription drugs now carry labels warning patients not to drink grapefruit juice or eat fresh grapefruit at the same time as taking the medicines.

But the new research shows that other drugs may be affected in the opposite way by fruit juices.

In tests, when the medicine fexofenadine - an antihistamine used to fight allergies - was taken with grapefruit juice, only half as much of the drug was absorbed into the body as it was with water.

Professor David Bailey, from the University of Western Ontario in London, Ontario, said losing half of a drug dose could be critical.

Grapefruit, orange and apple juices all lowered the absorption of the anti-cancer agent etoposide, he added.

They also reduced the potency of certain beta blockers, which are used to treat high blood pressure and prevent heart attacks.

The drugs include cyclosporine, a drug taken to prevent rejection of transplanted organs, and the antibiotics ciprofloxacin, levofloxacin and itraconazole.

Prof Bailey said: "This is just the tip of the iceberg. I'm sure we'll find more and more drugs that are affected in this way."

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