Grapefruit and medication: A cautionary note

Grapefruit can mean danger when combined with some popular drugs used for high blood pressure, high cholesterol and depression

Grapefruit and grapefruit juice are healthful, providing enough vitamin C, potassium, dietary fiber, and other nutrients to earn the American Heart Association's "heart-check" mark. That's the good news. The bad news is that grapefruit juice can interact with dozens of medications, sometimes dangerously.

Doctors are not sure which of the hundreds of chemicals in grapefruit are responsible. The leading candidate is furanocoumarin. It is also found in Seville (sour) oranges and tangelos; although these fruits have not been studied in detail, the guidelines for grapefruit should apply to them as well.

Why do grapefruit and medications interact? 

Grapefruit's culprit chemical does not interact directly with your pills. Instead, it binds to an enzyme in your intestinal tract known as CYP3A4, which reduces the absorption of certain medications. When grapefruit juice blocks the enzyme, it's easier for the medication to pass from your gut to your bloodstream. Blood levels will rise faster and higher than normal, and in some cases the abnormally high levels can be dangerous.

A variety of medications can be boosted by grapefruit juice; the table below lists some of the most important along with related drugs that are less likely to be influenced.

It doesn't take much grapefruit juice to boost the levels of drugs that are susceptible. A single glass can produce a 47% reduction of the intestinal enzyme that regulates absorption. And because this effect of the juice wears off slowly, a third of its impact is still evident after 24 hours.

What are the practical implications of this interaction? If you take one of the affected medications, the simplest solution is to switch to orange juice. If you are really hooked on grapefruit juice, though, you can ask your doctor whether you can switch to a related drug that's less vulnerable to the boosting effect. And if that's not possible, you should certainly avoid taking your pills and your juice simultaneously; the more time between the two, the better, and the smaller your glass of juice, the better. If you are on a low or moderate dose of the medication, you can probably get away with an occasional glass of grapefruit juice, but if you are on a high dose, it could be dangerous. That's especially true in the case of calcium channel blockers, which can lower your blood pressure or slow your heart rate excessively

Sildenafil (Viagra) is of special interest to men. The clinical information is incomplete, but men who take Viagra should be aware that grapefruit juice might boost blood levels of the drug. That could be a good thing for some men with erectile dysfunction, but it could trigger headaches, flushing, or low blood pressure.