Professor David Bailey of the University of Western Ontario addressed the 236th annual meeting of the American Chemical Society with the results of a study into how fruit juices react with medicines.

"Recently, we discovered that grapefruit and these other fruit juices substantially decrease the oral absorption of certain drugs undergoing intestinal uptake transport," said Professor Bailey.

"The concern is loss of benefit of medications essential for the treatment of serious medical conditions."

The study took healthy individuals and gave them fexofenadine, an antihistamine, along with either a glass of grapefruit juice, a glass of water containing naringin (the compound that makes grapefruit juice taste bitter), or water.

Grapefruit juice drinkers only absorbed half of the amount of fexofenadine, compared to those who drank water.

The water with naringin served to block a key drug uptake transporter called OATP1A2.

"Blocking this transporter reduces drug absorption and neutralizes their potential benefits," according to the study's findings.

"By contrast, drugs whose levels are boosted in the presence of grapefruit juice appear to block an important drug metabolising enzyme, called CYP3A4, that normally breaks down drugs."

Until more is known, patients are being advised to stick to water when taking medicines and to consult with their own doctor if they want to drink juice.

But Professor Bailey believes that much more research is required -- there could be many more drugs out there that interact badly with fruit juices.

"This is just the tip of the iceberg," he said.

"I'm sure we'll find more and more drugs that are affected this way."