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An antibiotic used by tens of thousands of Canadian every year can have a wide range of side effects – some of them serious – according to a report in the Canadian Medical Association Journal.

Trimethoprim-sulfamethoxazole, which has been on the market since 1968, is widely prescribed by doctors for treating urinary tract infections and other bacterial outbreaks because it is considered to be relatively cheap and effective.

But it is not without risks, warn the two authors of the article, Joanne Ho and David Juurlink of the University of Toronto. It can impair kidney function and trigger hypoglycemia, or low blood sugar, in some patients. Furthermore, it can interact with other medications including the blood thinner warfarin, ACE inhibitors for controlling high blood pressure, and a class of drugs called sulfonylureas, which are prescribed to patients with Type 2 diabetes.

In the case of warfarin, it essentially makes the blood thinner too effective, increasing the risk of excessive bleeding.

The researchers say physicians should be mindful of the drug's downside, especially when treating patients who are taking numerous medications.

"This antibiotic has saved many lives, but we need to take certain precautions," Dr. Ho said. She added that it may be appropriate to use another drug in some situations. But she noted other antibiotics can have unwanted side effects, too.

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