A. In the past decade, scientists have discovered that grapefruit juice has strong effects on many drugs. Grapefruit juice contains a substance that blocks enzymes in the intestines from regulating the amounts of certain medicines that enter the bloodstream. This causes excess amounts of some drugs to enter the blood. Grapefruit juice also slows down the rate at which these medicines are broken down. As a result, these medications stay in your system longer and can build up in the body to toxic levels.

The effects of grapefruit juice are quite strong. They can last up to 72 hours. This means it is not sufficient to merely avoid drinking grapefruit juice within a few hours of taking your medications.

It may be tempting to drink grapefruit juice as a way to decrease medication dosages or to supercharge your drugs effects. This is risky. Unlike medicines, grapefruit juice is not manufactured under controlled conditions. This means the amount of the active ingredient you get varies, making it impossible to predict the effect of a particular glass of grapefruit juice on drug metabolism.

Eating grapefruit sections may have less effect than a glass of juice. Even so, that effect still could be enough to cause you problems. In fact, grapefruit sections have been shown to have the same effect as juice on the blood-pressure drug Plendil. The bottom line: You should avoid all consumption of grapefruit and grapefruit juice if it interacts with your medicines.

Unlike grapefruit juice, orange juice does not inhibit the breakdown of drugs, so this would be a safe alternative.

Amy Tousman is a registered dietitian at Straub Clinic & Hospital Inc., and a member of the Hawai'i Dietetic Association.