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Why Grapefruit and Your Meds Don't Mix

While reading your prescription label, you see typical warnings: "may cause drowsiness, nausea and headaches," "do not take while operating heavy machinery," and the seemingly random "do not eat grapefruit while taking this medicine."

It's a disappointment to those who enjoy a glass of grapefruit juice when washing down their pills, but doctors have known for almost 15 years that this fruit can cause serious interactions with some medications. However, only now can they completely explain why grapefruit may be a pill taker's enemy.

Unlike other citrus juices, grapefruit juice contains furanocoumarins, a substance that seems to affect the way your body absorbs certain medications, including cholesterol-lowering drugs, some antihistamines, erectile dysfunction drugs and blood pressure medications.

Normally, these drugs are partially blocked from entering the body by an enzyme in the intestine. However, furanocoumarins prevent this enzyme from working, allowing potentially toxic levels of the drug to enter to body, which may cause organ damage.

In a study of the substance, researchers from the University of North Carolina at Chapel Hill gave 18 healthy volunteers felodipine, a common blood pressure lowering medication. Some took the drug with orange juice, some with grapefruit juice and the last group swallowed their pill with grapefruit juice that had the furanocoumarins removed.

Measuring the amount of medication in each patient's blood, the researchers discovered that those who used the furanocoumarin-free grapefruit juice had the same, normal amount of the drug in their blood as did those who used orange juice. The patients who drank normal grapefruit juice, however, had up to a 420 percent increase in blood levels of felodipine at any one time. The results of the study are published in the American Journal of Clinical Nutrition.

"This is the best evidence to date that furanocoumarins are the active ingredients in grapefruit juice that cause the interaction with medications," said Dr. Paul Watkins, lead study author.

Identifying this chemical, the researchers write, may lead to identifying other foods with furanocoumarins that may cause drug interactions. Also, for drugs that do not enter the body easily, furanocoumarins may be added to the pills to help them be absorbed by the intestine. Additionally, by removing this substance from grapefruit juice, it may be possible to create commercially-available grapefruit juice that has no medical concerns.

"Commercialization of a furanocoumarin-free grapefruit juice could provide an alternative for patients who are taking medications with interaction potential," writes Watkins.