Bactrim (Sulfamethoxazole-Trimethoprim) Dosage Guidelines
The standard adult dosage of Bactrim for urinary tract infections is 1 double-strength tablet (800 mg sulfamethoxazole/160 mg trimethoprim) or 2 regular-strength tablets (400 mg/80 mg) every 12 hours for 10 to 14 days. 1
Adult Dosing by Indication
Urinary Tract Infections
- 1 double-strength tablet (800 mg/160 mg) or 2 regular-strength tablets (400 mg/80 mg) every 12 hours for 10-14 days 1
- For single-dose therapy of uncomplicated UTIs in women: 2 double-strength tablets (1600 mg/320 mg) as a one-time dose 2
Shigellosis
- Same dosage as UTI but for 5 days: 1 double-strength tablet or 2 regular-strength tablets every 12 hours 1
Acute Exacerbations of Chronic Bronchitis
- 1 double-strength tablet or 2 regular-strength tablets every 12 hours for 14 days 1
Traveler's Diarrhea
- 1 double-strength tablet or 2 regular-strength tablets every 12 hours for 5 days 1
Pneumocystis jirovecii Pneumonia (PCP)
- Treatment: 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours, divided into equal doses every 6 hours for 14-21 days 1
- Prophylaxis: 1 double-strength tablet daily 1, 3
Pediatric Dosing
Children ≥2 months
- UTIs or Acute Otitis Media: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided into two doses every 12 hours for 10 days 1
- Shigellosis: Same dosage as above but for 5 days 1
- PCP Prophylaxis: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim given in equally divided doses twice daily, on 3 consecutive days per week (not to exceed 1600 mg sulfamethoxazole and 320 mg trimethoprim daily) 1
Dosing in Special Populations
Renal Impairment
- Creatinine clearance >30 mL/min: Standard regimen 1
- Creatinine clearance 15-30 mL/min: Half the usual regimen 1
- Creatinine clearance <15 mL/min: Not recommended 1
- For severe renal failure: Dosage interval (hours) should be increased to 12 times the serum creatinine level (mg/dL), maximum 48 hours 4
Elderly Patients
- Standard adult dosing, but monitor closely as pharmacokinetics may be altered 5
- Consider using doxycycline over fluoroquinolones for prophylaxis in elderly patients due to risk of QTc prolongation and other adverse effects with fluoroquinolones 3
- Be aware that plasma drug concentrations may be 2-3 times higher in elderly patients during continuous dosing 5
Administration Routes
Oral Administration
- Most common route for outpatient treatment 1
Intravenous Administration
- Available for patients unable to take oral medication 6
- Recommended loading dose: 250 mg trimethoprim and 1,250 mg sulfamethoxazole/m² 4
- Maintenance dose: 150 mg trimethoprim and 750 mg sulfamethoxazole/m² every 8 hours for children ≤10 years and every 12 hours for adults with normal renal function 4
Important Considerations
- Not recommended for children under 2 months of age 1
- Monitor for adverse effects including rash, gastrointestinal disturbances, and hematologic abnormalities 7
- Regular monitoring of complete blood counts with differential and platelet count is recommended, especially for long-term prophylactic use 7
- Thrombocytopenia risk increases with higher serum trimethoprim levels and longer treatment duration 4
- Adjust dosage for patients who are obese using adjusted body weight when prescribing doses >8 mg/kg/day 3