Why Grapefruit Affects Some Drugs

May 9, 2006 -- Researchers may have figured out why grapefruit and grapefruit juice interact with some types of drugs.

Grapefruit may interact with some drugs for cholesterol, blood pressure, heart rhythm, depression, anxiety, HIV, immunosuppression, allergies, impotence, and seizures. Grapefruit contains natural chemicals called furanocoumarins that may explain the interaction, at least with one particular drug, Plendil, scientists report in The American Journal of Clinical Nutrition.

It's not yet clear if furanocoumarins are also responsible for interactions between grapefruit and other drugs, note Mary Paine, PhD, and colleagues from the University of North Carolina at Chapel Hill.

If so, it might be possible to isolate furanocoumarins from grapefruit juice, the researchers write.

Three-Drink Trial

Paine's study included 18 healthy, nonsmoking adults who were in their mid to late 30s, on average.

After carefully screening participants (including pregnancy tests for the nine women enrolled in the study), the researchers asked participants to avoid products containing grapefruit.

At least a week later, participants reported to the research center after fasting overnight. They took a 10-milligram tablet of Plendil with a glass of regular grapefruit juice, orange juice, or furanocoumarin-free grapefruit juice.

Plendil is a calcium channel blocker, a type of drug used to treat heart disease and high blood pressure.

Participants tried one drink per session. Sessions were held at least a week apart.

The researchers made the furanocoumarin-free grapefruit juice in their lab. They checked to make sure it was free of furanocoumarins. The drink was "sweeter and less bitter" than the regular grapefruit juice, write Paine and colleagues.

Blood Tests

Participants stayed at the research center overnight. Every few hours, the researchers checked participants' blood samples for Plendil levels.

As expected, maximum blood concentrations of Plendil were higher when the drug was taken with regular grapefruit juice. Blood concentrations of Plendil didn't spike with orange juice - which doesn't react with Plendil like grapefruit juice -- or furanocoumarin-free grapefruit juice.

The Plendil and juice treatments were "generally well tolerated," the researchers note. They add that none of the participants commented on the taste of the furanocoumarin-free grapefruit juice.

The study only tested Plendil and grapefruit juice.

If furanocoumarins prove to be responsible for other grapefruit-drug reactions, creating commercially available furanocoumarin-free grapefruit juice "could provide an alternative for patients who are taking medications with interaction potential," write Paine and colleagues.

Check with your doctor about potential interactions between grapefruit and any drugs you take.