Grapefruit

Citrus fruits are an important part of a good diet because they provide vitamin C and other essential nutrients. There  is approximately 65 mg of vitamin C and 250 mg of potassium in an orange. Grapefruit contain high levels of a flavonoid ( plant compound) called naringin. This substance reduces the activity of a group of P450 enzymes known as CYP3A enzymes. Your body uses these enzymes to break down certain drugs, such as calcium channel blockers ( commonly used in the treatment of high blood pressure), sedatives, and cyclosporine ( an immune suppressant given to people who have received organ transplants). If you eat a lot of grapefruit, the naringin may inhibit the action of these enzymes. The drugs, if taken concurrently, are not metabolized. They remain in the body in  concentrations higher than normal. This increases the risk of unwanted toxic effects.

If you are taking prescription medication, ask your physician if you should avoid eating grapefruit or grapefruit juice. Certain drugs, like cyclosporine, already carry a warning.

The good news is that there are plenty of substitutes. Orange, tangerines, and tangelos have plenty of anti-oxidants but very low levels of naringin.