What is it about grapefruit juice that affects some drugs?

Researchers in the United States say they have identified the compound in grapefruit juice that affects how some drugs are absorbed in the body and produces unwanted side effects.

They believe their discovery may lead to some patients being able to lower the dosages of their medications.

It is common for patients to be advised by their doctors to avoid drinking grapefruit juice while taking a variety of medications because of the potentially dangerous side effects.

The juice should be avoided when taking certain blood-pressure or cholesterol-lowering drugs, as well as HIV medications, organ-transplant drugs and sedatives.

It had originally been thought that the flavonoids that give grapefruit juice its bitter taste were what caused the drug interaction, but lead researcher Dr. Paul Watkins, director of the General Clinical Research Center at the University of North Carolina at Chapel Hill, says they found that a group of substances called furanocoumarins, when removed from the juice, destroys this particular effect.

Grapefruit juice is known for its effects on drug metabolism and is avoided by some patients while other deliberately take their drugs with the juice.

It seems an intestinal enzyme called CYP3A, which partially destroys drugs as they are absorbed in the body is affected by grapefruit juice so the body ends up absorbing more of the drug.

In their study, Watkins and his colleagues compared orange juice, whole grapefruit juice and grapefruit juice that had the furanocoumarins removed.

They found that once the furanocoumarins were removed the juice behaved like orange juice.

Watkins suggests there are three implications to the finding.

Researchers can now look at a number of other fruits to see if they contain furanocoumarins and predict whether they will cause the same problem.

Secondly, manufacturers of grapefruit juice could offer a version of the juice that has the furanocoumarins removed, eliminating the potential for drug interaction.

Thirdly, furanocoumarins could be added to certain drugs to improve their "bioavailability", or ability to enter the blood stream.

This is probably the most interesting possibility.

The study findings appear in the current issue of the American Journal of Clinical Nutrition.