TORONTO - Eating a grapefruit or drinking its juice can be a great way to get vitamin C, but it can also be dangerous when taking certain prescription drugs, researchers warn.

Grapefruit juice can interact with more than 85 oral medications, with about half of them potentially leading to severe — even deadly — consequences, finds a comprehensive review by a research team affiliated with Western University in London, Ont.

"Many of the drugs that interact with grapefruit are highly prescribed and are essential for the treatment of important or common medical conditions," authors of the review write in Monday's issue of the Canadian Medical Association Journal.

Lead researcher David Bailey, a clinical pharmacologist at the Lawson Health Research Institute, said that in the last four years, the number of medications with the potential to interact with grapefruit and cause severe adverse effects has increased from 17 to at least 43.

"Half of these drugs actually can cause sudden death," Bailey said from London, Ont., noting that interactions can result in acute kidney or respiratory failure, gastrointestinal bleeding, or other serious effects.

Medications altered by grapefruit include highly prescribed cholesterol-lowering statins, some key heart drugs, and certain anti-psychotic and pain medicines.

That's because grapefruit contains a chemical that interferes with an enzyme that controls how drugs are absorbed through the intestines, resulting in a potentially toxic dose of medication.

"We know it boosts drug levels in blood," Bailey said. "Now you're seeing so many drugs where the levels get boosted that the consequences are really quite dire."

Other citrus fruits that contain the chemical, at least to some degree, include limes, pomelos and Seville oranges, an ingredient frequently used in marmalades.

Commonly prescribed statins like atorvastatin (Lipitor), simvastatin (Zocor) and lovastatin (Mevacor, Altoprev) have a high to very-high risk of interacting with grapefruit and causing rhabdomyolysis, a breakdown of muscle fibres that can lead to acute kidney failure.

"Your urine looks like wine," said Bailey. "You can be in such extreme pain that you literally can't walk across the room and your kidneys have totally shut down.

"You don't have to take an awful lot of grapefruit. You can be taking one grapefruit a day or just drinking 250 millilitres. But the magnitude of the increase (of the drug) in some individuals can be so large that you can precipitate this adverse event."

The anti-clotting agent clopidogrel (Plavix), taken to prevent a heart attack or stroke, "doesn't work at all if grapefruit is eaten," he said, pointing out another medication often taken by older Canadians.

Bailey was part of a research group that first raised a red flag about a handful of grapefruit-drug interactions more than 20 years ago, and since then the number of affected medications coming to market has jumped significantly.

"But the biggest problem that we have facing us is that the science is there, but it sits on the shelf; it's not being applied, which is very frustrating," he said.

"It's gone from this curiosity — 'It's interesting, but what does it mean in real practice?' — to the point where everybody needs to know about this."

Bailey hopes that by alerting doctors through the CMAJ article, they will make sure patients prescribed an affected drug will be told not to ingest grapefruit or its juice.

As well, the information could help physicians trying to diagnose a serious health event in a patient without knowing grapefruit is part of their diet, he said, and where the person hasn't thought to mention the fact.

"That's why we wrote the paper. If you're not aware of the possibility, it's very unlikely you'll think of it."

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Online: www.cmaj.ca/lookup/doi/10.1503/cmaj.120951

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