Bactrim Dosing for Adults
For most adult infections, the standard dose is one double-strength tablet (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily, with duration varying by indication from 3 days for uncomplicated cystitis to 14 days for bronchitis. 1
Standard Dosing by Common Indications
Urinary Tract Infections
- Uncomplicated cystitis in women: 160/800 mg (one double-strength tablet) twice daily for 3 days when local E. coli resistance is <20% 2
- Complicated UTIs: One double-strength tablet or two single-strength tablets every 12 hours for 10-14 days 1
Respiratory Infections
- Acute exacerbations of chronic bronchitis: One double-strength tablet every 12 hours for 14 days 1
Skin and Soft Tissue Infections (including MRSA)
- One to two double-strength tablets twice daily (total daily dose: 320-640 mg trimethoprim and 1600-3200 mg sulfamethoxazole) for 7-10 days 2
Gastrointestinal Infections
- Shigellosis: One double-strength tablet every 12 hours for 5 days 1
- Traveler's diarrhea: One double-strength tablet every 12 hours for 5 days 1
Pneumocystis Pneumonia (PCP)
Treatment Dosing
- 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim, divided into four doses every 6 hours for 14-21 days 1
- For a 70 kg adult, this translates to approximately 2 double-strength tablets every 6 hours (upper limit dosing) 1
Prophylaxis Dosing
- One double-strength tablet daily for HIV-infected adults with CD4+ counts <200 cells/µL 2
- Alternative schedule: One double-strength tablet three times weekly on consecutive days (equally effective) 2
Intravenous Dosing
- Trimethoprim 320 mg and sulfamethoxazole 1,600 mg per day, divided into 2 doses every 12 hours 2
Renal Dose Adjustments
Dosing must be reduced in renal impairment to prevent toxicity:
- Creatinine clearance >30 mL/min: Standard dosing 1
- Creatinine clearance 15-30 mL/min: Half the usual regimen 1
- Creatinine clearance <15 mL/min: Use not recommended 1
For severe renal failure, the dosage interval in hours should be increased to 12 times the serum creatinine level in mg/dL (maximum 48 hours) 3
Special Population Considerations
Obese Patients
- Calculate dose using adjusted body weight for BMI ≥30, though standard prophylactic dosing does not require adjustment 2
Elderly Patients
- Elderly patients achieve 2-3 times higher steady-state concentrations than younger adults due to reduced renal clearance 4
- Increased risk of acute kidney injury and hyperkalemia, especially with concurrent ACE inhibitors, ARBs, or potassium-sparing diuretics 2
- Monitor renal function and potassium levels closely 2
Critical Safety Monitoring
Hematologic Monitoring
- Perform complete blood counts monthly during prophylaxis, as neutropenia and thrombocytopenia correlate directly with serum drug concentrations 2
- Thrombocytopenia risk increases with higher serum trimethoprim levels and longer treatment duration 3
Hydration Requirements
- Ensure adequate hydration to prevent crystalluria and renal stones 2
Drug Interactions
- Avoid concurrent use with methotrexate at treatment doses due to severe bone marrow suppression risk (prophylactic methotrexate doses may be tolerated) 2