How does grapefruit interact with drugs?

Grapefruit is considered a very Healthy food because it's a good source of fiber, vitamins A and C, potassium and antioxidants — but there are times when people should avoid the sour citrus. If a person is taking certain medications, then eating whole grapefruit or drinking grapefruit juice could lead to serious side effects or make the medicine work less effectively.

But why does grapefruit interact with certain medications, and what are examples of drugs people shouldn't take with grapefruit?

The primary culprit behind these drug interactions is a group of chemicals in grapefruit called furanocoumarins, Patrick McDonnell, professor of clinical pharmacy at Temple University School of Pharmacy in Pennsylvania, told Live Science in an email. Furanocoumarins can mess with some molecules and enzymes — proteins that perform chemical reactions in the body — in the small intestine.

These molecules and enzymes are responsible for breaking down many drugs and carrying them into the bloodstream, but furanocoumarins throw them out of whack. In some instances, the furanocoumarins can effectively boost the dose of a drug in the body, leading to unintended effects.

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When medication is swallowed, it is broken down, or metabolized, by enzymes in the small intestine. One key enzyme is cytochrome P450 3A4, or CYP3A4. As the furanocoumarins in grapefruit are broken down in the body, their resulting derivatives can block the action of CYP3A4. Thus, instead of being broken down as usual, more of the drug enters the blood and also stays in the body longer, potentially leading to unwanted effects.

This can happen with certain blood-pressure medications, such as nifedipine (brand name Procardia and Adalat CC). These drugs work by relaxing blood vessels to lower blood pressure, but consuming grapefruit can increase the amount of the drug in the body, causing blood pressure to drop too low. This chain of events can also lead to a too-slow heart rate, or bradycardia, McDonnell said.