People taking the popular blood-thinner warfarin (often sold as Coumadin) should avoid an antibiotic that's often prescribed to treat urinary tract infections.
Researchers with the Ontario-based Institute for Clinical Evaluative Sciences found that warfarin can be dangerous when combined with the antibiotic cotrimoxazole, sold under such brand names as Bactrim and Septra.
ICES researchers conducted a study which found patients on warfarin who were also prescribed cotrimoxazole had an almost four-fold greater risk of bleeding in their upper-gastrointestinal tract, compared to those not given the antibiotic.
The 10-year-study looked at more than 134,000 older Ontarians who were taking warfarin, which is used to prevent blood clots that could cause a heart attack or stroke. They found:
- About 34 per cent were prescribed at least one prescription for antibiotics used to treat UTI.
- Seven per cent of patients received at least one prescription for cotrimoxazole.
- More than 2,000 patients treated with warfarin were hospitalized with bleeding from the stomach and duodenum.
- In patients taking warfarin, use of cotrimoxazole was associated with an almost four-fold increase in the risk of upper gastrointestinal (UGI) hemorrhage compared to no antibiotic use.
The study is published in the Archives of Internal Medicine.
The researchers say the risk of bleeding is considerably higher with cotrimoxazole than that from other antibiotics. Fro that reason they urge doctors to avoid prescribing cotrimoxazole to patients taking warfarin.
Principal investigator Hadas Fischer says in those rare instances when the two drugs must be used together, doctors should monitor patients very closely.
"This is a completely avoidable problem," says co-author Dr. David Juurlink, a scientist at ICES. "Whenever possible, clinicians should prescribe alternative antibiotics to cotrimoxazole in patients receiving warfarin."
Urinary-tract infections are extremely common among the elderly and often are treated with antibiotics.
