Drinking fruit juice dramatically reduces the effectiveness of drugs used to treat cancer, heart conditions and high blood pressure, scientists say.

Research has shown that orange, apple and grapefruit juice can also wipe out the benefits of some antibiotics and hay-fever pills.

It is thought the drinks stop drugs from entering the bloodstream and getting to work in the body  -  possibly rendering them useless.

The potential effects are so serious, researchers warned, that if in doubt, patients should swap fruit juices for water when on medication.

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

Professor Bailey, of the University of Western Ontario, in Canada, said: 'The concern is loss of benefit of medications essential for the treatment of serious medications.'

Drugs shown to be weakened by grapefruit, orange and apple juices include the blood pressure-lowering beta blockers atenolol, celiprolol, and talinolol and the hay-fever treatment fexofenadine.

The multi-purpose antibiotic ciprofloxacin, used to combat germs behind food poisoning and bone and joint infections, is also affected.

So is the cancer drug etoposide and a drug given to prevent the rejection of transplanted organs.

Many other drugs are also likely to be affected, an American Chemical Society conference heard yesterday.

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