Q: The label on my grapefruit juice says that it's heart healthy. But the label on the cholesterol-lowering drug I take, Lipitor, reads, "Do not eat grapefruit or drink grapefruit juice at any time while taking this medication." Why?

A: Grapefruit and grapefruit juice definitely qualify as healthful. They contain enough vitamin C, potassium, dietary fiber and other nutrients to earn the American Heart Association's "heart-check" mark. There's even some evidence that the tart fruit may help lower blood pressure and cholesterol levels. It's also fairly popular - 21 percent of all households in the United States buy grapefruit juice. That's the good news. The bad news is that grapefruit juice can interact with dozens of medications (including Lipitor) - sometimes with dangerous results. The accompanying table lists some of the most important ones, along with related drugs that are less likely to be influenced.

Grapefruit contains certain compounds called flavonoids that can change the way certain drugs are absorbed by your body. These particular flavonoids are also found in tangelos (a cross between a grapefruit and a tangerine) and Seville (sour) oranges, which are used to make marmalade. Although these fruits have not been studied in detail, the guidelines for grapefruit should apply to them as well. Other citrus fruits, such as oranges, limes and tangerines, don't contain the compounds.

Inside our intestines, special enzymes break down drugs before they reach the bloodstream. The flavonoid compounds in grapefruit block the effects of these enzymes. When that happens, it's easier for certain drugs to pass from your intestines to your bloodstream. So levels of those drugs rise faster and higher than normal. As an example, the amount of Lipitor absorbed into the blood stream is 20 percent to 50 percent higher if the drug is taken with grapefruit juice. That means if you take a 10-milligram (mg) dose of Lipitor, you could get the equivalent of 12 to 15 mg of Lipitor if you took if with grapefruit juice. High levels can increase the risk of side effects, such as muscle damage. However, these types of interactions seem to be rare.

It doesn't take much grapefruit juice to boost the levels of drugs that are susceptible. A single glass can cut the effect of the special enzyme that regulates absorption by nearly half. Not everyone will have a problem, however, because the amount of the enzyme we have in our intestinal wall varies. Unfortunately, there is no easy way to tell if you are one of the people who have a lot or a little of the enzyme.

In general, if you take a low or moderate dose of a medication affected by grapefruit juice, you can probably get away with an occasional grapefruit half or glass of 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. Another drug of special concern for men is sildenafil (Viagra). The clinical information is still 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 in others.

To be on the safe side, switch to another fruit and juice. But if you don't want to give up your grapefruit juice, you might talk to your doctor about switching to a different cholesterol-lowering drug.

Submit questions to the Harvard Medical School Adviser at www.health.harvard.edu/adviser. Unfortunately, personal responses are not possible.

POST A COMMENT

If you are under 13 years of age you may read this message board, but you may not participate. Here are the full legal terms you agree to by using this comment form.