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Warning issued about mixing blood thinners and some antibiotics

Certain antibiotics used to treat urinary tract infections can trigger dangerous bleeding in patients taking blood thinners, researchers are warning.

Clinicians and patients alike should be particularly cautious about combining warfarin and the antibiotic cotrimoxazole, according to the findings.

The research, published in today's edition of the Archives of Internal Medicine, shows that the combination of the two drugs results in a four-fold increase in the risk of upper gastrointestinal bleeding.

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The study, conducted among Ontario residents over the age of 65 found that, over a 10-year period, 34 per cent had taken warfarin (sold under the brand name Coumadin and various generic brands) and 7 per cent had taken cotrimoxazole (brand names Septra and Bactrim).

"Both are very common medications and the risk that they would be combined is very real," said Dr. David Juurlink, a senior scientist at the Institute for Clinical Evaluative Sciences and co-author of the study.

During the study period, 2,151 of the participants were hospitalized for treatment of upper gastrointestinal bleeding within two weeks of taking an antibiotic.

It is well-established that antibiotics can interact with warfarin. That is because, generally speaking, blood thinners work by blocking the effect of vitamin K in the gut. Antibiotics, for their part, interfere with the bacteria in the gut that make vitamin K.

So researchers set out to compare which antibiotics posed the greatest risk by looking at those who suffered severe GI bleeds shortly after starting antibiotic treatment for infections.

The study showed that seniors on blood thinners who took cotrimoxazole in the two weeks preceding hospitalization to treat a GI bleed had a risk that was 4.5 time higher than those who did not take antibiotics.

Those who took the antibiotic ciprofloxacin had a risk that was smaller, but still significant at 1.9 times higher.

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Those taking other common antibiotics such as amoxicillin, ampicillin, nitrofurantoin, norfloxacin and ocular antibiotics, had small or negligible increase in risk.

"The message here is that not all antibiotics are created equal," said Dr. Juurlink, who is also head of the division of clinical pharmacology and toxicology at Sunnybrook Health Sciences Centre in Toronto.

He said that, in some cases, cotrimoxazole may still be the best treatment but, if patients are taking blood thinners, they should be monitored closely.

Cotrimoxazole is a combination of two drugs, sulfamethoxazole and trimethoprim.

Warfarin is used to prevent blood clots, particularly in patients with a history of heart attack or stroke, those suffering from chronic obstructive pulmonary disease, deep vein thrombosis and those with mechanical heart valves. It is a commonly prescribed drug, with Canadian sales of $81.5-million last year, according to IMS Health Canada.

Warfarin is also a drug that interacts with a long list of prescription drugs, natural health products and foods.

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Most dangerous of all is combining warfarin with acetaminophen (brand name Tylenol) or acetylsalicylic acid (ASA, commonly known by its brand name, Aspirin).

Warfarin can also interact with antibiotics, antidepressants, statins, natural health products such as coenzyme Q10, ginseng, vitamin K and even foods like avocados, flax and onions.

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