A fixed-dose combination antibiotic containing sulfamethoxazole, a sulfonamide, and trimethoprim, a diaminopyrimidine. This synergistic combination sequentially inhibits two consecutive steps in the bacterial folate synthesis pathway, providing broad-spectrum bactericidal activity against a wide range of Gram-positive and Gram-negative organisms. It is a cornerstone of therapy for urinary tract infections, acute otitis media, acute exacerbations of chronic bronchitis, traveler's diarrhea, and Pneumocystis jirovecii pneumonia (PCP) prophylaxis and treatment in India.
Adult: For most infections: 1 tablet (SMX 800mg/TMP 160mg) every 12 hours. For PCP treatment: 15-20 mg/kg/day (based on TMP) in 3-4 divided doses. For PCP prophylaxis: 1 tablet (SMX 800mg/TMP 160mg) daily or 1 DS tablet three times per week.
Note: Administer with a full glass of water. Maintain adequate fluid intake to prevent crystalluria. Can be taken with or without food; taking with food may reduce gastrointestinal upset. Complete the full prescribed course even if symptoms improve.
The combination acts synergistically by sequentially inhibiting two enzymes in the bacterial tetrahydrofolic acid (THF) synthesis pathway. Sulfamethoxazole competitively inhibits dihydropteroate synthase, preventing the incorporation of para-aminobenzoic acid (PABA) into dihydrofolic acid. Trimethoprim is a competitive inhibitor of bacterial dihydrofolate reductase (DHFR), preventing the reduction of dihydrofolic acid to tetrahydrofolic acid. This dual blockade depletes THF, a crucial cofactor for the synthesis of purines, thymidine, and amino acids, leading to bacteriostatic and, at high concentrations, bactericidal effects.
Pregnancy: Category D (US FDA). Contraindicated at term. Trimethoprim is a folate antagonist; theoretical risk of teratogenicity (neural tube defects) especially in first trimester. Use only if potential benefit justifies potential fetal risk. Folinic acid supplementation may be considered.
Driving: Use with caution. May cause dizziness, headache, or fatigue, impairing ability to drive or operate machinery.
| Warfarin | Increased anticoagulant effect, risk of bleeding | Major |
| Phenytoin | Increased phenytoin levels, risk of toxicity | Major |
| Methotrexate | Increased methotrexate levels and bone marrow toxicity | Major |
| Sulfonylureas (e.g., Glimepiride) | Enhanced hypoglycemic effect | Moderate |
| ACE Inhibitors (e.g., Ramipril) | Increased risk of hyperkalemia | Moderate |
| Cyclosporine | Increased risk of nephrotoxicity and reduced cyclosporine levels | Moderate |
| Thiazide Diuretics | Increased risk of thrombocytopenia in elderly | Moderate |
| Rifampin | Increased clearance of trimethoprim, reducing efficacy | Moderate |