a sulfonamide (trade name Gantanol) used to treat infections (especially infections of the urinary tract)

Sulfamethoxazole is a sulfonamide bacteriostatic antibiotic. It is most often used as part of a synergistic combination with trimethoprim in a 5:1 ratio in co-trimoxazole, which is also known as Bactrim or Septrin. It can be used as an alternative to amoxicillin-based antibiotics to treat sinusitis.