We all know, or at have heard, that drinking grapefruit juice can increase the absorption of certain oral drugs. The result of the increased levels of absorption is the possibility that normal doses of drugs become toxic overdoses. At the 236th National Meeting of the American Chemical Society there was a presentation that provided new evidence that grapefruit and other common fruit juices, including orange and apple, can have the opposite effect. It seems that these juices can substantially decrease the absorption of other drugs, potentially wiping out their beneficial effects.

We now have a reason to avoid drinking grapefruit juice as well as these other juices when taking certain drugs. The drugs that are affected are the drugs prescribed for heart disease, cancer, organ-transplant rejection, and infection. These findings come from a controlled human study conducted by David G. Bailey, Ph.D., a professor of clinical pharmacology with the University of Western Ontario in London, Ontario, which was designed to evaluate the drug-juice interaction.

Dr. Bailey said, “Recently, 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.”

Bailey’s original research, almost 20 years ago, demonstrated that grapefruit juice could dramatically boost the body’s absorption levels of the high-blood-pressure drug felodipine, causing blood toxicity issues. Other researchers, in the intervening time, have identified around 50 additional medications that also carry the risk of grapefruit-induced drug-overdose interactions. This “Grapefruit Juice Effect,” now causes some prescription drugs to carry warning labels against taking grapefruit juice or fresh grapefruit during their consumption.

In the most recent research, volunteers were given fexofenadine, an antihistamine used to fight allergies. The subjects consumed the drug with either a single glass of grapefruit juice, water containing only naringin (substance in grapefruit juice that gives the juice its bitter taste), or water. When fexofenadine was taken with grapefruit juice, only half of the drug was absorbed compared to taking the drug with water alone, Bailey says. Loosing half of the amount of drugs taken into the body can be critical for the performance certain drugs, he points out.

Additionally, the naringin blocked a key drug uptake transporter, called OATP1A2. OATP1A2 moves the drug from the small intestine to the bloodstream, blocking it reduces drug absorption and neutralizes their potential benefits. By contrast, those drugs whose levels are boosted in the presence of grapefruit juice appear to block a metabolizing enzyme, called CYP3A4 that normally breaks down drugs.

“This is just the tip of the iceberg,” Bailey says. “I’m sure we’ll find more and more drugs that are affected this way.”

Our important take home message is to consult your doctor or pharmacist before taking any medications with grapefruit juice or any other fruits and juices. Dr. Bailey recommends that unless it is known to be a problem, he recommend taking most medications only with water to avoid either under or over absorption of your treatment drugs.

Joel T Nowak MA, MSW