Considerations when prescribing trimethoprim-sulfamethoxazole.

famethoxazole remains a popular antibiotic because of its low cost, effectiveness and familiarity among clinicians. It is the most frequently prescribed antibiotic for urinary tract infections in Canada. Other indications include treatment of infections caused by Pneumocystis jiroveci, Toxoplasma gondii, Stenotrophomonas maltophilia and community-associated methicillin-resistant Staphylococcus aureus. In addition, among patients with depressed CD4 counts from infection with HIV, the use of low-dose trimethoprim–sulfamethoxazole for prophylaxis against P. jiroveci and T. gondii is associated with decreased mortality caused by opportunistic infections. With up to 4000 prescriptions for trimethoprim–sulfamethoxazole dispensed each week in Ontario, this drug is used by hundreds of thousands of Canadians each year. More than 40 years of widespread use has provided ample opportunity to identify the many adverse events associated with trimethoprim– sulfamethoxazole. Although this drug is well tolerated by many patients, it is associated with several potentially serious adverse reactions. Most of these associations are supported only by case reports and case series, but some have been the subject of volunteer studies and observational studies. Many of these adverse effects are rare, however others are predictable and several can be life-threatening. Here we provide an overview of the various toxicities associated with the use of trimethoprim–sulfamethoxazole and offer a simple mnemonic — “NOT RISKY?” — to help clinicians recall the various toxicities associated with this drug (Table 1).