[6] It is primarily used in the treatment of urinary tract infections, although it may be used against any susceptible aerobic bacterial species.

[7] It is generally not recommended for the treatment of anaerobic infections such as Clostridioides difficile colitis (the leading cause of antibiotic-induced diarrhea).

[10][11] It may be involved in a reaction similar to disulfiram when alcohol is consumed after it is used, in particular when used in combination with sulfamethoxazole.

A 2013 study found a doubling in the risk of miscarriage in women exposed to trimethoprim in the early pregnancy.

[25] Sulfamethoxazole inhibits dihydropteroate synthase, an enzyme involved further upstream in the same pathway.

[24] Trimethoprim and sulfamethoxazole are commonly used in combination due to possible synergistic effects, and reduced development of resistance.