TORONTO - A commonly used antibiotic can trigger a wide range of rare but potentially serious side-effects and doctors should be aware of those risks when prescribing it, a report published Tuesday warns.

The drug, trimethoprim-sulfamethoxazole, is the most commonly prescribed antibiotic in Canada for urinary tract infections. It also is used to treat a potentially fatal type of pneumonia in HIV patients.

Between 3,000 and 4,000 prescriptions for the drug are written each week in Ontario alone.

But while the antibiotic is a lifesaver in many circumstances, it can trigger potentially fatal drug interactions in others, Dr. Joanne Ho and Dr. David Juurlink warned in a review of the drug published in the Canadian Medical Association Journal.

For instance, the drug can impede the body's ability to break down warfarin, a commonly used blood thinner. That means people taking trimethoprim-sulfamethoxazole while on warfarin could have their blood thinned too much, putting them at risk of developing dangerous bleeds.

The antibiotic can also interact with a class of drugs called sulfonylureas that are used in the treatment of Type 2 diabetes, and with ace inhibitors used to treat high blood pressure.

Ho said the point of the review is not to discourage the use of trimethoprim-sulfamethoxazole, but to remind doctors of the risks associated with it.

It should be used with caution in older patients, she said, as they often are on multiple medications and are therefore at high risk of drug interactions.

Ho and Juurlink work at Toronto's Sunnybrook Health Sciences Centre.