Grapefruit – the forbidden fruit?

Slices of red grapefruit

Many people think that a fresh grapefruit for breakfast is a healthy way to kick-start the day, but in fact a single grapefruit or glass of grapefruit juice could have serious consequences for those who take daily medication for a number of causes.

The reason for this is that grapefruit contains chemicals which affect the rate at which certain drugs are broken down in the body. A specific enzyme found in the lining of the small intestine and colon, and also in some liver cells, is responsible for breaking down about 50% of all medications. The chemicals in grapefruit inhibit this enzyme and so prevent it from breaking the drug down in the usual way, which leads to more of the drug staying in the circulation than the dose prescribed. One study showed that a single 250 ml glass of grapefruit juice could increase the concentration of felodipine, a blood pressure-lowering medication, by up to 8 times – the equivalent of taking a staggering 8 doses of the medication all at once. Another study showed that a 200 ml glass of grapefruit juice once a day for three days increased the concentration of simvastatin, a cholesterol-lowering drug, by 330% and the concentration of tacrolimus, an anti-rejection drug used after organ transplantation, was found to be 500% of normal following a large intake of grapefruit marmalade in the preceding week. Concentrations at levels such as these can, depending on the drug in question, cause kidney failure, liver failure and even sudden death from fatal changes in heart rhythm.

Many avid grapefruit lovers are keen to know if there if there is any time of day that they could safely consume half a grapefruit or a glass of juice, but really the safest thing is to avoid it altogether if there is any risk of interaction. When doctors looked at felodipine and grapefruit juice they found that the biggest reaction came when the tablet was taken 4 hours after a glass of grapefruit juice. At 10 hours’ difference the effect was 50% of the maximum, and at 24 hours it was still 25% of the maximum – if the maximum possible reaction were 800%, that would mean that even a whole day after a glass of grapefruit juice, the concentration of drug in the body could still reach 200% of the prescribed dose. The effect of a single glass of grapefruit can actually last up to three days, hence why patients taking medication known to interact with it are advised to avoid it entirely.  This advice also applies to Seville oranges, limes and some types of citrus marmalades; the sweet varieties of oranges and lemons are safe, however.

There are estimated to be around 85 different drugs that may interact with grapefruit. Below we have listed some of the more common cardiovascular medications known to interact, but this list is not exhaustive and so if you are a grapefruit lover, do check with your doctor or pharmacist before tucking in.

Drug  Drug Use  Potential adverse event
 Amiodarone  To control an abnormal heart rhythm  Palpitations, dizziness, collapse, sudden death
 Amlodipine To lower blood pressure   Very low blood pressure, swelling of the extremities
 Apixaban  Blood thinner  Gastric bleeding
 Atorvastatin  To lower cholesterol  Severe muscle pain, kidney failure
 Felodipine  To lower blood pressure  Very low blood pressure, swelling of the extremities
 Nifedipine  To lower blood pressure  Very low blood pressure, swelling of the extremities
 Rivaroxaban  Blood thinner  Gastric bleeding
 Simvastatin  To lower cholesterol  Severe muscle pain, kidney failure
 Verapamil  To lower blood pressure  Very low blood pressure, swelling of the extremities
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