1. Clinical Overview
Trimethoprim is a synthetic, broad-spectrum antibacterial agent primarily used as a dihydrofolate reductase inhibitor. It is a key component in the treatment of uncomplicated urinary tract infections (UTIs) and is often used in combination with sulfamethoxazole (as cotrimoxazole) for synergistic effect. As a monotherapy, the 200mg strength is a standard dose for acute cystitis in women. It is particularly relevant in the Indian context due to its efficacy against common uropathogens like E. coli, cost-effectiveness, and wide availability.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are achieved within 1-4 hours after oral administration. | The antibacterial effect persists for approximately 8-12 hours, supporting a twice-daily dosing regimen. | 90-100% |
2. Mechanism of Action
Trimethoprim selectively and competitively inhibits bacterial dihydrofolate reductase (DHFR). This enzyme is responsible for converting dihydrofolic acid to tetrahydrofolic acid, a crucial cofactor in the synthesis of thymidine, purines, and amino acids. Inhibition depletes thymidine, leading to impaired bacterial DNA synthesis and cell death.
3. Indications & Uses
- Acute Uncomplicated Cystitis in Women
- Initial episodes of Uncomplicated Lower Urinary Tract Infections (UTIs)
- Prophylaxis of recurrent UTIs (at a lower dose, e.g., 100mg nightly)
4. Dosage & Administration
Adult Dosage: For Uncomplicated UTI: 200 mg orally every 12 hours for 3-7 days (standard Indian course is often 5 days). For Prophylaxis: 100 mg orally once daily at bedtime.
Administration: Administer with or without food. Take with a full glass of water. Maintain adequate fluid intake to ensure proper hydration and urinary output. Complete the full prescribed course even if symptoms improve earlier.
5. Side Effects
Common side effects may include:
- Nausea
- Vomiting
- Epigastric discomfort
- Pruritus (itching)
- Skin Rash (usually maculopapular)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Warfarin | Increased anticoagulant effect; risk of bleeding | Major |
| Phenytoin | Increased phenytoin levels and risk of toxicity | Moderate |
| Digoxin | May increase digoxin serum concentration in some patients | Moderate |
| ACE Inhibitors (e.g., Ramipril) / ARBs / Potassium-Sparing Diuretics | Additive risk of hyperkalemia | Major |
| Methotrexate | Increased risk of methotrexate toxicity and bone marrow suppression | Major |
| Procainamide | Increased procainamide levels | Moderate |
| Rifampin | Decreased trimethoprim levels; reduced efficacy | Moderate |
| Cyclosporine | May increase risk of nephrotoxicity | Moderate |
7. Patient Counselling
- DO complete the full course of medication as prescribed.
- DO take with plenty of water and maintain good fluid intake.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DO report any signs of rash, sore throat, fever, or unusual bleeding/bruising immediately.
- DONT skip doses. If you miss a dose, take it as soon as you remember, but skip if it's almost time for the next dose.
- DONT take antacids containing magnesium or aluminum simultaneously; separate by 2 hours if needed.
- DONT use for viral infections like common cold or flu.
8. Toxicology & Storage
Overdose: Acute overdose may present with nausea, vomiting, dizziness, headache, mental confusion, bone marrow depression (evident as fever, sore throat, pallor, purpura), and hyperkalemia. Crystalluria is rare with trimethoprim alone.
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.