Does a grapefruit a day keep the doctor away?

Or does grapefruit cause more harm than good when taken with medication?

You might know grapefruit as the fun, citrusy cousin of the lemon or lime. Grapefruits are certainly a healthy fruit, full of vitamin C, antioxidants, and fiber. However, grapefruits are hiding a secret property that is relatively unknown- chemicals in grapefruits can interact harmfully with some medications.

Photo by Estúdio Bloom on Unsplash

The idea that grapefruit can interact with certain medications has been around since 1989, when Bailey and colleagues discovered this phenomenon by accident as part of  a clinical trial. The trial was conducted to determine if there are any dangerous interactions between alcohol and a blood pressure medication, felodipine. As part of the trial, the scientists used grapefruit juice to hide the taste of alcohol. However, they found that the grapefruit juice caused an increase in the amount of felodipine in the patients’ blood. This was unexpected, as there was not an identifiable reason for the phenomena. Scientists eventually realized this effect was due to the type of juice that was used in the trial.

More work has been done to determine how grapefruit juice affects medication after the initial discovery that consuming grapefruit can result in increased levels of drug within blood. Drug metabolism, or the critical process of breaking down drugs so they can be removed from the body, is carried out by a type of protein called enzymes. When an enzyme is blocked, it cannot break down the drug that it targets in the body. As a result, there will be higher levels of the drug in your body. This may cause a patient to need hospitalization to treat the effects of the drug.

The cytochrome P450 class of enzymes is responsible for breaking down and removing drugs from the body; 52% of drugs are broken down in the body by P450 enzymes. Researchers discovered that grapefruit juice can interfere with the enzyme cytochrome P450 3A4 (CYP3A4), which is found in the liver. The isoform CYP3A4 is a specific variant of the P450 class of enzymes, and is one of the most prominent drug metabolizing enzymes. This is due to the large active site where many different sizes of molecules can fit for breakdown and removal from the body through the liver.

Recent research has shown that the chemical in grapefruit that causes this effect are known furanocoumarins. Furanocoumarins are common in citrus fruits like grapefruit. They have been linked to health benefits such as anticancer activity. However, these compounds still pose a threat when taken with certain medications. Specifically, drugs that are metabolized by the enzyme CYP3A4 may be affected by grapefruit juice. This includes common over the counter drugs like the allergy medicine Allegra, blood pressure drugs, and drugs that lower cholesterol. 

Grapefruit juice is now listed by the Food and Drug Administration (FDA) as a substance that can interfere with medication. So, how can we be aware of this potential issue with grapefruit and our medications? The FDA has developed resources to help determine if a drug might be affected by grapefruit. It is also important to talk with your doctor or pharmacist about any potential harmful interactions between your medications and common foods.

Peer editor: Jeanne-Marie McPherson