1. Clinical Overview
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.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are achieved within 1-4 hours after oral administration. | Approximately 12-24 hours, supporting a twice-daily or single-dose regimen for certain indications. | 90-100% following oral administration. |
2. Mechanism of Action
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.
3. Indications & Uses
- Acute, uncomplicated urinary tract infections (cystitis) in women
- Prophylaxis of recurrent urinary tract infections
4. Dosage & Administration
Adult Dosage: 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.
Administration: Administer with or without food. Take with a full glass of water. Maintain adequate fluid intake to ensure proper hydration and urinary output.
5. Side Effects
Common side effects may include:
- Nausea
- Vomiting
- Diarrhea
- Loss of appetite
- Pruritus (itching)
- Skin rash (maculopapular)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| 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 |
7. Patient Counselling
- DO complete the full prescribed course even if you feel better.
- DO drink plenty of water (2-3 liters daily) while on this medication.
- DO take it at the same time each day if on a prophylactic regimen.
- DO inform your doctor if you are taking any blood pressure (ACE-i/ARB) or blood thinner (warfarin) medicines.
- DONT take antacids containing magnesium or aluminum simultaneously; space by 2 hours.
- DONT use it for viral infections like cold or flu.
8. Toxicology & Storage
Overdose: Nausea, vomiting, dizziness, headache, mental confusion, bone marrow depression (evidenced by thrombocytopenia, leukopenia, megaloblastic anemia), and crystalluria (rare).
Storage: Store below 30°C in a cool, dry place, protected from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.