Sulfamethoxazole (C₁₀H₁₁N₃O₃S, CAS 723-46-6) is a broad-spectrum sulfonamide antibiotic used to treat bacterial infections such as urinary tract infections, bronchitis, and ear infections. It inhibits bacterial folate synthesis by competing with p-aminobenzoic acid (PABA). Often combined with trimethoprim to block sequential steps in folate metabolism, enhancing activity against pathogens like Pneumocystis jirovecii in immunocompromised patients. Biotech grade (purity ≥98.5%, conforms to NMR) for laboratory research, cell culture, pharmaceutical development, and veterinary applications.