Septra (Trimethoprim-Sulfamethoxazole) 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 to 14 days. 1
Standard Adult Dosing by Indication
Urinary Tract Infections
- Uncomplicated cystitis: One double-strength tablet twice daily for 3 days when local E. coli resistance is <20% 2
- Complicated UTI: One double-strength tablet (or two single-strength tablets) every 12 hours for 10-14 days 1
Skin and Soft Tissue Infections (MRSA)
- One to two double-strength tablets twice daily (320-640 mg TMP/1600-3200 mg SMZ total daily dose) 3, 4
- Duration typically 7-10 days depending on clinical response 3
Respiratory Infections
- Acute exacerbations of chronic bronchitis: One double-strength tablet every 12 hours for 14 days 1
- 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
- High-dose therapy required: 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours, divided every 6 hours for 14-21 days 1
- For a 70 kg adult, this translates to 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 4, 2
- Alternative schedule: One double-strength tablet three times weekly on consecutive days (equally effective) 4, 2
Renal Dose Adjustments
Critical adjustment required for impaired renal function 1:
- CrCl >30 mL/min: Standard dosing
- CrCl 15-30 mL/min: Reduce dose by 50% (half the usual regimen)
- CrCl <15 mL/min: Use not recommended 1
- Hemodialysis patients requiring prophylaxis: 500 mg three times weekly after dialysis 4
Special Population Considerations
Obesity
- For obese adults (BMI ≥30) receiving doses >8 mg/kg/day, calculate using adjusted body weight: ABW = IBW + ([TBW-IBW] × 0.4) 3
- Standard prophylactic dosing does not require adjustment 3
Elderly Patients
- Increased risk of acute kidney injury and hyperkalemia, especially when combined with ACE inhibitors, ARBs, or potassium-sparing diuretics 3
- Monitor renal function and potassium levels closely 3
Critical Safety Monitoring
Hematologic Toxicity
- Obtain baseline complete blood count and monitor monthly during prolonged therapy 4, 2
- Risk of thrombocytopenia, neutropenia, and leukopenia increases with duration of therapy 2, 5
Hypoglycemia Risk
- Renal insufficiency is the primary risk factor (present in 93% of reported cases) 6
- Mean daily dose associated with hypoglycemia was 4.5 double-strength tablets per day 6
- Mechanism appears sulfonylurea-like with inappropriately elevated insulin levels 6
Drug Interactions
- Avoid concurrent use with methotrexate at treatment doses due to severe bone marrow suppression risk 3, 4
- Lower prophylactic doses (one double-strength tablet 3× weekly) are generally tolerated with methotrexate 3
Absolute Contraindications
- Pregnancy (especially at term) 4, 2
- Nursing mothers 2
- Infants <2 months of age (kernicterus risk) 2, 1
- G6PD deficiency (hemolytic anemia risk) 4
Common Pitfalls to Avoid
- Failure to adjust for renal function: Hypoglycemia and hematologic toxicity increase dramatically without dose reduction 6
- Inadequate hydration: Ensure adequate fluid intake to prevent crystalluria and renal stones 2
- Ignoring drug interactions in elderly: Particularly dangerous with ACE inhibitors/ARBs causing hyperkalemia 3
- Using for remission maintenance in GPA: Methotrexate or azathioprine are preferred over trimethoprim-sulfamethoxazole for this indication 3