Grapefruit Juice

Grapefruit – one of those fruits you either love or hate – was first grown in Barbados in about 1750 where it was known as the ‘forbidden fruit’. If you don’t like the fruit, then you don’t need to read any further. Like other fresh citrus fruits, it is a good source of vitamin C. 

The problem of drugs interacting with grapefruit juice was first discovered by accident in the late 1980’s when the juice was given to flavour a medicine during a test. The scientists found that the effects of the medicine under test were much greater than they had anticipated and, after several more tests, they found that grapefruit was the culprit.

Many medications are broken down by enzymes called CYP3A4 in the cells lining in the intestines and in the liver. When working normally, these enzymes break down some of these drugs into harmless substances. Fresh grapefruit juice stops these enzymes from working. This leads to the drug being passed directly into the blood stream without being broken down. At these higher concentrations, the drugs can cause problems – effectively, it is like taking an overdose of the drug, leading to potentially serious side effects.

The list of medicines that are affected by grapefruit is quite long - about 50 different drugs can be affected - and include blood thinning tablets, blood pressure lowering medicines, cholesterol lowering drugs, anti-convulsants, drugs that keep the heart rhythm under control as well as drugs used to treat HIV infection amongst others. As you can see, many of them are very commonly used so you need to read the patient information sheet inside every box of medicine very carefully to ensure that you are not putting yourself at any un-necessary risk.

Other fruits that can cause similar effects include pomelos and Seville oranges. The effects of the fruit can take up to 24 hours or more to wear off, so if you are on long term tablets that are known to be affected by grapefruit, you will need to avoid it completely. Most other juices, including orange juice are safe.

As always, if you are unsure what you should do, talk to your doctor or pharmacist who can give you more information about what you can and cannot take with your medicine.

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