Trimethoprim is a synthetic, broad-spectrum antibacterial agent primarily used as a single agent for the treatment of uncomplicated urinary tract infections (UTIs) and as a component of combination therapy (with sulfamethoxazole) for various infections. It is a dihydrofolate reductase inhibitor, selectively targeting bacterial folate synthesis. The 300mg strength is a standard adult dose for UTI treatment in India.
Adult: For acute uncomplicated UTI: 300mg once daily for 3-5 days. For prophylaxis of recurrent UTI: 100mg at bedtime or 300mg three times a week.
Note: Administer with or without food. Take with a full glass of water. Maintain adequate fluid intake to ensure proper hydration and urinary output.
Trimethoprim selectively and competitively inhibits bacterial dihydrofolate reductase (DHFR). This enzyme is responsible for converting dihydrofolic acid to tetrahydrofolic acid, an essential co-factor for the synthesis of thymidine, purines, and amino acids. Inhibition disrupts bacterial DNA, RNA, and protein synthesis, leading to bacteriostatic action.
Pregnancy: Pregnancy Category C (US FDA). Animal studies show teratogenicity (cleft palate). Use during pregnancy, especially first trimester, only if potential benefit justifies potential fetal risk. Not recommended for treatment of asymptomatic bacteriuria in pregnancy.
Driving: Unlikely to affect ability. However, patients experiencing rare CNS side effects like dizziness should avoid driving.
| Warfarin | Increased anticoagulant effect, elevated INR risk | Major |
| Phenytoin | Increased phenytoin levels, risk of toxicity | Major |
| Digoxin | Possible increase in digoxin serum levels (especially in elderly) | Moderate |
| ACE Inhibitors (e.g., Ramipril) / ARBs (e.g., Telmisartan) | Increased risk of hyperkalemia | Major |
| Dofetilide | Contraindicated - trimethoprim inhibits renal secretion, increasing dofetilide levels and risk of torsades de pointes | Contraindicated |
| Methotrexate | Increased risk of methotrexate toxicity and blood dyscrasias | Major |
| Procainamide | Increased procainamide and NAPA levels, risk of toxicity | Moderate |
| Cyclosporine | May increase risk of nephrotoxicity | Moderate |
| Rifampicin | Decreased trimethoprim levels, reduced efficacy | Moderate |