What is the recommended dosage and treatment duration for trimethoprim-sulfamethoxazole (Bactrim) in bacterial infections?

Trimethoprim-Sulfamethoxazole (Bactrim) Dosing and Duration

For most bacterial infections, the standard adult dose of Bactrim is 1-2 double-strength tablets (800mg/160mg) twice daily, with treatment duration ranging from 3 days for uncomplicated cystitis to 14-21 days for severe infections like Pneumocystis pneumonia. 1, 2, 3

Adult Dosing by Infection Type

Urinary Tract Infections

• Uncomplicated cystitis in women: 1 double-strength tablet twice daily for 3 days 2, 3
• Complicated UTI: 1 double-strength tablet twice daily for 10-14 days 3
• Pyelonephritis: 1 double-strength tablet twice daily for 14 days (only if pathogen is known to be susceptible) 2

Critical caveat: Do not use Bactrim empirically if local E. coli resistance exceeds 20%, as treatment failure risk increases >17-fold with resistant strains 2

Skin and Soft Tissue Infections

• Uncomplicated SSTI/MRSA infections: 1-2 double-strength tablets twice daily for 7-10 days 1, 2
• Severe infections requiring IV therapy: 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses 1

Important note: Bactrim is particularly effective against MRSA but has limited efficacy against streptococci in non-purulent cellulitis, where beta-lactams are preferred 1

Respiratory Infections

• Acute exacerbations of chronic bronchitis: 1 double-strength tablet twice daily for 14 days 3
• Pneumocystis jirovecii pneumonia (treatment): 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours divided every 6 hours for 14-21 days 3
• PCP prophylaxis: 1 double-strength tablet daily 3

Other Infections

• Shigellosis: 1 double-strength tablet twice daily for 5 days 3
• Traveler's diarrhea: 1 double-strength tablet twice daily for 5 days 3

Pediatric Dosing (≥2 months of age)

Bactrim is NOT recommended for infants under 2 months of age 3

Standard Pediatric Dosing

• UTI and acute otitis media: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided every 12 hours for 10 days 3
• Shigellosis: Same dose for 5 days 3
• PCP treatment: 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours divided every 6 hours for 14-21 days 3
• PCP prophylaxis: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim divided twice daily on 3 consecutive days per week 3

Weight-Based Tablet Dosing (every 12 hours)

• 22 lbs (10 kg): 1 tablet 3
• 44 lbs (20 kg): 1 tablet 3
• 66 lbs (30 kg): 1½ tablets 3
• 88 lbs (40 kg): 2 tablets or 1 double-strength tablet 3

Prophylactic Regimens

For bacterial respiratory tract infection prophylaxis in immunodeficiency:

• Children: 5 mg/kg trimethoprim daily or twice daily 4
• Adults: 160 mg trimethoprim daily or twice daily 4

This is particularly important for patients with severe T-cell deficiency or dysfunction 4

Renal Dose Adjustment

Dosing must be reduced in renal impairment: 3

• CrCl >30 mL/min: Standard dosing
• CrCl 15-30 mL/min: Half the usual regimen
• CrCl <15 mL/min: Use not recommended

Critical Safety Considerations

Contraindications and Warnings

• Pregnancy: Category C/D; NOT recommended in third trimester due to increased risk of birth defects 1, 2
• Sulfa allergies: Consider alternative agents 1
• Immunocompromised patients: Only irradiated, CMV-negative blood products should be used if transfusion needed 4

Monitoring Requirements

• Regular CBC monitoring recommended for prolonged therapy due to risk of bone marrow suppression 2
• Clinical improvement should be evident within 48-72 hours 2

Common Adverse Effects

• Gastrointestinal disturbances, rash, and photosensitivity are common 2
• Serious reactions include Stevens-Johnson syndrome, toxic epidermal necrolysis, and bone marrow suppression 2

When to Consider Alternatives

Switch to alternative antibiotics when: 1, 2

• Local resistance rates exceed 20% for UTIs
• Non-purulent cellulitis (streptococcal coverage needed)
• Mixed aerobic-anaerobic wound infections (Bactrim lacks anaerobic coverage)
• Sulfa allergy present
• Third trimester pregnancy

Alternative agents include: 2

• Clindamycin 300-450 mg orally three times daily
• Fluoroquinolones (when local resistance <10%)
• Nitrofurantoin for uncomplicated cystitis only
• Beta-lactams (though generally inferior efficacy for UTIs)

Clinical Pearls

For severe infections requiring hospitalization, initiate IV therapy before transitioning to oral treatment 2. Aggressive and prolonged antimicrobial therapy should be considered for immunodeficient patients, as standard regimens may be inadequate 4. Rising trimethoprim-sulfamethoxazole resistance among uropathogens may limit first-line use in some regions 2.