David Bailey, a clinical pharmacologist at the Lawson Health Research Institute in London, Ontario, first discovered the interaction between grapefruit and certain medications more than 20 years ago. Since then the number of drugs with the potential to interact has jumped.
"What I've noticed over the last four years is really quite a disturbing trend and that is the increase in the number of drugs that can produce not only adverse reactions but extraordinarily serious adverse drug reactions," Bailey told the Canadian Broadcasting Corp.
Many of the drugs are common, such as some cholesterol-lowering statins, antibiotics and calcium channel blockers used to treat high blood pressure. Other medications include agents used to fight cancer or suppress the immune system in people who have received an organ transplant, Bailey said.
People age 45 and older buy the most grapefruit and take the most prescription drugs, making this group the most likely to face interactions, Bailey said.
The study, published in the Canadian Medical Association Journal, said of the 85 known drugs that interact with grapefruit, 43 can have serious side-effects, including sudden death, acute kidney failure, respiratory failure, gastrointestinal bleeding and bone marrow suppression in people with weakened immune systems.
"Taking one tablet with a glass of grapefruit juice is like taking 20 tablets with a glass of water," Bailey said. "This is unintentional overdosing. So it's not surprising that these levels go from what we call therapeutic to toxic."
All sources of grapefruit -- the whole fruit or a glass of grapefruit juice -- and other citrus fruit such as Seville oranges, often used in marmalade, limes and pomelos can lead to drug interactions.