Grapefruit and grapefruit juice can change the way some medicines work, the FDA says, and it requires warnings on certain prescription and over-the-counter drugs taken by mouth. Not every drug in an affected category interacts, and the effect depends on the person, the drug and the amount of juice.

Grapefruit itself can be part of a healthy diet, the FDA notes, since it has vitamin C and potassium. The concern is what it does to certain drugs. With most affected drugs, "the juice lets more of the drug enter the blood," says Shiew Mei Huang, Ph.D., of the FDA. "When there is too much drug in the blood, you may have more side effects." The FDA gives certain cholesterol-lowering statins as an example, since too much of the drug may stay in the body.

The FDA explains that grapefruit juice blocks an enzyme in the intestine called CYP3A4 that breaks down these drugs. People have different amounts of the enzyme, so the juice can affect two people differently even when they take the same drug.

A few drugs go the other way. The FDA says grapefruit juice can cause less fexofenadine, the allergy drug sold as Allegra, to enter the blood, so it may not work as well. Orange and apple juice can do the same, and its label says, "Do not take with fruit juices."

The FDA suggests asking your doctor or pharmacist whether grapefruit affects your medicine, reading the information that comes with a prescription drug, and checking the Drug Facts label on over-the-counter drugs. If you must avoid grapefruit, it adds, check juice labels, and skip Seville oranges, pomelos and tangelos, which might have the same effect.