It is most often used as part of a synergistic combination with trimethoprim in a 5:1 ratio in co-trimoxazole (abbreviated SMZ-TMP and SMX-TMP, or TMP-SMZ and TMP-SMX), also known under trade names such as Bactrim, Septrin, or Septra; in Eastern Europe it is marketed as Biseptol. Its primary activity is against susceptible forms of Streptococcus, Staphylococcus aureus (including MRSA), Escherichia coli, Haemophilus influenzae, and oral anaerobes. It is commonly used to treat urinary tract infections. In addition it can be used as an alternative to amoxicillin-based antibiotics to treat sinusitis. It can also be used to treat toxoplasmosis and it is the drug of choice for Pneumocystis pneumonia, which affects primarily patients with HIV.
Other names include: sulfamethylisoxazol, sulfisomezole, MS 53, RO 4 2130, and sulfamethazole.