MedWire News: People with hayfever and other allergies should avoid drinking grapefruit fruit juice at the same time as taking antihistamines, say researchers who found that the juice reduces the effectiveness of these medications.
Twenty years ago, lead researcher Dr David Bailey, from the University of Western Ontario in London, Canada, and colleagues found that grapefruit juice can dramatically increase the body's absorption of high-blood-pressure drugs, potentially causing dangerous effects from excessive drug concentrations in the blood.
Since then, other researchers have identified many more medications that interact with grapefruit juice. As a result, some prescription medications now carry warning labels against drinking or eating grapefruit while taking the drugs.
In the current study, Dr Baily and team studied healthy volunteers who were asked to take the antihistamine fexofenadine while drinking grapefruit juice, water containing naringin - the substance in grapefruit juice that gives it a bitter taste - or water alone.
They found that participants who drank grapefruit juice or water with naringin absorbed only half the amount of the antihistamine as that absorbed by the participants who only drank water.
The researchers also found that naringin appears to block a key drug uptake transporter, called OATP1A2, which is involved in transporting the drugs from the small intestine into the bloodstream.
"This is just the tip of the iceberg," said Dr Bailey. "I'm sure we'll find more and more drugs that are affected this way."
He added that grapefruit juice may not be the only type of fruit juice that affects drug absorption, as orange and apple juices also contain naringin-like substances that may inhibit OATP1A2.
Speaking at the National Meeting of the American Chemical Society in Philadelphia, Pennsylvania, USA, Dr Bailey concluded: "We discovered that grapefruit and these other fruit juices substantially decrease the oral absorption of certain drugs undergoing intestinal uptake transport.
"The concern is loss of benefit of medications essential for the treatment of serious medical conditions."