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.

Related: What's the difference between fruit and vegetables?

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.

Grapefruit can also have this effect on some cholesterol-lowering statins, such as simvastatin (Zocor) and atorvastatin (Lipitor). Too-high doses of these statins can lead to muscle pain and damage. According to the Food and Drug Administration (FDA), grapefruit can also boost the levels of certain corticosteroids used to treat inflammatory bowel diseases, such as Crohn's disease.

Grapefruit can also boost the levels of immunosuppressants, such as cyclosporine (Neoral), which can potentially harm the kidneys. A similar effect on anxiety medications, including benzodiazepines like diazepam (Valium), can lead to excessive drowsiness, McDonnell noted. Depending on the dose of benzos a person has taken, there could be more severe effects, such as difficulty breathing.

But grapefruit doesn't exert this boosting effect on all the drugs it messes with — in some cases, it can actually decrease the amount of a drug that enters the blood.

This is true of the allergy medication fexofenadine (Allegra). Drugs like fexofenadine are moved into cells by proteins called transporters, which move substances from one side of a cell's membrane to the other. Some of these transporters help move drugs from the intestines into the blood. Grapefruit-borne furanocoumarins block certain transporter proteins, thus causing too little of the drug to enter the bloodstream.

The FDA notes that the effects of grapefruit can vary depending on the amount of the fruit or juice a person has consumed and the medication they took. But the effects can be dramatic.

"Even just one grapefruit or an 8-ounce [236 milliliters] glass of grapefruit juice can change how your body processes certain drugs, especially those that rely on CYP3A4 for breakdown," McDonnell said.

There are other fruits that interact with drugs, "but none really are quite as dramatic as seen with grapefruit juice," McDonnell noted. This is probably because grapefruit contains a higher concentration of furanocoumarins compared to other fruits. But other furanocoumarin-containing fruits — such as Seville oranges, tangelos and pomelos — should still be treated with caution, according to Harvard Health.

Besides furanocoumarins, the citric acid in fruits can pose its own issues. The osteoporosis drug alendronate (Fosamax) interacts with various citrus juices, including orange and grapefruit. If ingested at the same time, the citric acid in the juices decreases absorption of the drug, making it less effective, McDonnell said.

Interactions with grapefruit and other foods should be listed in the information leaflet that comes with a prescription or over-the-counter medication. Harvard Health also has a short list of medications that interact with grapefruit and possible alternative drugs that do not. The FDA advises asking your doctor or pharmacist to see if grapefruit juice interacts with your medication.

This article is for informational purposes only and is not meant to offer medical advice.

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