What is the recommended dosing of double‑strength Bactrim (trimethoprim 160 mg/sulfamethoxazole 800 mg) for adults, when is a double‑dose regimen indicated, and how should the dose be adjusted for renal impairment, elderly patients, and children?

Bactrim Double-Strength Dosing Recommendations

Standard Adult Dosing

For most common infections in adults, administer 1 double-strength tablet (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily, though severe infections may require 2 double-strength tablets twice daily. 1

Indication-Specific Dosing

Urinary Tract Infections & Shigellosis:

• Standard dose: 1 double-strength tablet every 12 hours for 10-14 days (UTI) or 5 days (shigellosis) 1
• Single-dose therapy (2 double-strength tablets as a single dose) achieves 93% cure rates for uncomplicated UTI in women with significantly fewer side effects (4% vs 24%) compared to conventional 10-day therapy 2

Skin and Soft Tissue Infections (including MRSA):

• 1-2 double-strength tablets twice daily for typically 7 days 3
• Critical caveat: Bactrim has poor activity against beta-hemolytic streptococci; do not use as monotherapy for non-purulent cellulitis where streptococci are likely 3

Acute Exacerbations of Chronic Bronchitis:

• 1 double-strength tablet every 12 hours for 14 days 1

Pneumocystis jirovecii Pneumonia (PCP):

• Treatment: 15-20 mg/kg/day trimethoprim (75-100 mg/kg/day sulfamethoxazole) divided every 6 hours for 14-21 days 1
  • For an 80 kg adult: 5 double-strength tablets daily (2.5 tablets every 12 hours or 1.25 tablets every 6 hours) 1
• Prophylaxis: 1 double-strength tablet daily OR 1 double-strength tablet three times weekly on consecutive days (equivalent efficacy with fewer side effects) 4, 3

Severe Infections Requiring IV Therapy:

• 8-12 mg/kg/day (based on trimethoprim) divided into 4 doses, each infused over 1 hour 3
• Transition to oral therapy using the same total daily dose once clinical improvement occurs in patients without malabsorption 3

Renal Impairment Dosing

Dose adjustment is mandatory when creatinine clearance falls below 30 mL/min: 1

• CrCl >30 mL/min: Standard dosing 1
• CrCl 15-30 mL/min: Reduce dose by 50% (½ the usual regimen) 1
• CrCl <15 mL/min: Use not recommended per FDA label 1, though clinical evidence shows it can be used cautiously with appropriate monitoring 5, 6

Hemodialysis Patients:

• ½ double-strength tablet after each dialysis session (three times weekly) 7
• For active infections: 1 double-strength tablet after each dialysis session 7
• Key principle: Always administer post-dialysis to minimize toxicity 7

Important pharmacokinetic consideration: Both trimethoprim and sulfamethoxazole accumulate when CrCl <30 mL/min, but this does not preclude use with appropriate dose reduction 5, 6

Elderly Patients

Use standard dosing but monitor closely for adverse effects: 8

• Trimethoprim peak concentrations are 30% higher and AUC is 44% larger in elderly patients compared to young adults 8
• Renal clearance of trimethoprim is reduced by approximately 65% in elderly patients 8
• Steady-state plasma concentrations during continuous dosing are 2-3 times higher than after single doses 8
• Practical approach: Consider starting at standard doses but monitor renal function closely and adjust based on creatinine clearance using the renal dosing algorithm above 8

Pediatric Dosing

Children ≥2 months of age: 1

• UTI/Acute Otitis Media/Shigellosis: 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim divided every 12 hours 1

  • Weight-based tablet dosing:
    • 10-20 kg: 1 single-strength tablet every 12 hours 1
    • 30 kg: 1.5 single-strength tablets every 12 hours 1
    • 40 kg: 1 double-strength tablet every 12 hours 1

• PCP Prophylaxis: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim divided twice daily, given 3 consecutive days per week (maximum 1,600 mg sulfamethoxazole/320 mg trimethoprim daily) 1

Absolute contraindication: Children <2 months of age 1

Critical Safety Considerations

Contraindications:

• Third trimester pregnancy (kernicterus risk) 3
• Nursing mothers 3
• G6PD deficiency (hemolytic anemia risk) 4
• Sulfa allergies 3

Monitoring Requirements:

• Baseline hemogram and monthly monitoring for hematological toxicity, particularly thrombocytopenia 4
• Avoid concurrent methotrexate at treatment doses (severe bone marrow suppression risk) 4
• Watch for drug interactions with anticoagulants and antidiabetic agents 7

Common Pitfall: Do not use Bactrim as monotherapy for mixed aerobic-anaerobic wound infections due to lack of anaerobic coverage 3