What is the recommended dose of Bactrim (trimethoprim/sulfamethoxazole) for adults?

Bactrim Dosing for Adults

Standard Adult Dosing

For most common infections in adults, Bactrim is dosed as 1-2 double-strength tablets (800mg sulfamethoxazole/160mg trimethoprim) twice daily by mouth. 1

Specific Indications and Dosing

Uncomplicated Urinary Tract Infections and Shigellosis:

• 1 double-strength tablet (or 2 single-strength tablets) every 12 hours 1
• Duration: 10-14 days for UTI; 5 days for shigellosis 1

Skin and Soft Tissue Infections (Purulent Cellulitis/MRSA):

• 1-2 double-strength tablets twice daily 2, 3
• Duration: typically 7 days based on clinical response 3
• This is an AII-level recommendation from the Infectious Diseases Society of America for purulent cellulitis where MRSA is suspected 2, 3

Acute Exacerbations of Chronic Bronchitis:

• 1 double-strength tablet (or 2 single-strength tablets) every 12 hours for 14 days 1

Traveler's Diarrhea:

• 1 double-strength tablet (or 2 single-strength tablets) every 12 hours for 5 days 1

Pneumocystis jirovecii Pneumonia (Treatment):

• 15-20 mg/kg/day trimethoprim component in divided doses every 6 hours for 14-21 days 1
• For an 80 kg adult, this translates to approximately 5 double-strength tablets every 6 hours (or 2.5 double-strength tablets every 6 hours for the lower dose range) 1

Pneumocystis jirovecii Pneumonia (Prophylaxis):

• 1 double-strength tablet daily 1

Severe Infections Requiring IV Administration

For CNS infections (meningitis, brain abscess) or severe bacteremia:

• 5 mg/kg/dose (based on trimethoprim) IV every 8-12 hours 2
• This is a BIII/CIII recommendation for MRSA CNS infections 2

For complicated infections requiring parenteral therapy:

• 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses IV 3

Renal Dose Adjustment

Bactrim requires dose reduction in renal impairment: 1

• Creatinine clearance >30 mL/min: Use standard dosing 1
• Creatinine clearance 15-30 mL/min: Reduce dose by 50% 1
• Creatinine clearance <15 mL/min: Use not recommended 1

Critical Clinical Considerations

Activity Limitations:

• Bactrim has poor activity against beta-hemolytic streptococci, so it should not be used alone for non-purulent cellulitis where streptococci are likely pathogens 2, 3
• For non-purulent cellulitis, beta-lactams are preferred, and Bactrim should only be added if MRSA is suspected and the patient fails beta-lactam therapy 2

Pregnancy and Contraindications:

• Pregnancy Category C/D—contraindicated in third trimester due to kernicterus risk 2
• Contraindicated in nursing mothers 2
• Avoid in patients with sulfa allergies 3

Common Pitfalls:

• Do not use Bactrim as monotherapy for mixed aerobic-anaerobic wound infections—it lacks anaerobic coverage and requires combination therapy 3
• In elderly patients, plasma concentrations can be 2-3 times higher at steady state, and renal clearance of trimethoprim is significantly reduced, increasing risk of toxicity 4
• Side effects occur in approximately 24% of patients on conventional 10-day therapy versus only 4% with single-dose therapy for uncomplicated UTI 5