Grapefruits stop some drugs from working

By Clint Witchalls

BMJ Group News

What do we know already?

Grapefruits, Seville oranges, and limes contain substances called furanocoumarins that stops certain drugs from working properly. Furanocoumarins interfere with an enzyme in your digestive system (called CYP3A4) that stops some drugs from getting into the body and being able to work properly.

Pharmaceutical companies that manufacture drugs take this into account when making new drugs. They know that only a small proportion of the drug we take will be absorbed by our bodies. This is known as the drug’s bioavailability. If the enzyme is prevented from doing its job by substances such as grapefruits, too much of the drug enters your body, potentially causing serious health problems.

Grapefruit interferes with a wide range of drugs, including some antidepressants, antihistamines, antiretrovirals, blood pressure drugs, cancer drugs, and cholesterol-lowering drugs. For example, a single glass of grapefruit juice (200 ml to 250 ml), on average, increases the strength of a blood pressure-lowering drug called felodipine by three times.

In this latest study, researchers reviewed published studies of grapefruit-drug interactions to see how many drugs could cause this problem.

What does the new study say?

The number of new drugs that have a dangerous interaction with grapefruit - reactions such as internal bleeding, kidney failure, and an irregular heartbeat - has more than doubled in the past four years.

In 2008, 17 drugs were known to have a dangerous interaction with grapefruit and other citrus fruits. By 2012, that number had risen to 43.

The researchers said there are 85 prescription drugs available in Canada that could potentially interact with grapefruit. Many of these drugs are also available in the UK.

How reliable is the research?

The research is a review of all the known studies that have been published on grapefruit-drug interactions. The review included 102 randomised controlled trials, which is the best type of study for finding out if one thing causes another. Although the research is reliable, we still don’t know how common this problem is.

What does this mean for me?

If you take prescription drugs, check with your doctor or pharmacist before eating grapefruit products or related citrus products. Grapefruit continues to interfere with drugs for at least 24 hours after consumption.

Not all citrus fruits contain furanocoumarins. For example, sweet oranges such as Valencia don’t contain this substance and are unlikely to interfere with any prescribed medication.