What is the recommended dosage of Bactrim (trimethoprim/sulfamethoxazole) for treating bacterial infections?

Bactrim (Trimethoprim-Sulfamethoxazole) Dosing Guidelines

The standard adult dosage of Bactrim DS (double-strength) for treating bacterial infections is 1-2 double-strength tablets (800mg/160mg) twice daily for 7-14 days, depending on the type and severity of infection. 1

Adult Dosing by Infection Type

Urinary Tract Infections

• 1 double-strength tablet (800mg/160mg) or 4 teaspoons (20mL) of oral suspension every 12 hours for 10-14 days 2
• For acute uncomplicated cystitis in women, treatment duration can be shortened to 3 days 3
• For pyelonephritis, 1 double-strength tablet twice daily for 14 days is recommended if the pathogen is known to be susceptible 3

Skin and Soft Tissue Infections

• 1-2 double-strength tablets twice daily for 7-10 days 1
• For MRSA infections, 1-2 double-strength tablets twice daily is effective 3

Acute Exacerbations of Chronic Bronchitis

• 4 teaspoons (20mL) of oral suspension or 1 double-strength tablet every 12 hours for 14 days 2

Traveler's Diarrhea

• 1 double-strength tablet every 12 hours for 5 days 2

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 2
• Prophylaxis: 1 double-strength tablet daily 2

Pediatric Dosing

• For children ≥2 months: 8-12 mg/kg/day (based on trimethoprim component) in 2 divided doses 2
• Weight-based dosing guide:
  • 10 kg (22 lb): 1 teaspoon (5 mL) every 12 hours 2
  • 20 kg (44 lb): 2 teaspoons (10 mL) every 12 hours 2
  • 30 kg (66 lb): 3 teaspoons (15 mL) every 12 hours 2
  • 40 kg (88 lb): 4 teaspoons (20 mL) every 12 hours 2
• For UTIs and acute otitis media: treatment duration is typically 10 days 2
• For shigellosis: treatment duration is 5 days 2
• Bactrim is contraindicated in infants under 2 months of age 2

Dose Adjustments

Renal Impairment

• Creatinine clearance >30 mL/min: standard regimen 2
• Creatinine clearance 15-30 mL/min: reduce dose by 50% 2
• Creatinine clearance <15 mL/min: not recommended 2

Special Considerations

Pregnancy

• Not recommended for women in the third trimester of pregnancy (pregnancy category C/D) 1
• Can increase risk of birth defects if used near term 1

Elderly Patients

• May require dose adjustment due to decreased renal function 4
• Elderly patients have higher peak concentrations and larger area under the curve for trimethoprim compared to younger adults 4

Monitoring and Adverse Effects

• Common side effects include gastrointestinal disturbances, rash, and photosensitivity 1
• Serious adverse reactions include Stevens-Johnson syndrome, toxic epidermal necrolysis, and bone marrow suppression 1
• Regular monitoring of complete blood count is recommended for prolonged therapy 1
• Clinical improvement should be evident within 48-72 hours of starting appropriate therapy 1

Alternative Agents

• For patients who cannot tolerate TMP-SMX or in cases of resistance, consider:
  • Clindamycin (300-450 mg orally three times daily) 3
  • Fluoroquinolones (when appropriate and resistance is <10%) 3
  • Nitrofurantoin for uncomplicated cystitis (not for pyelonephritis) 3
  • β-lactams as alternative options, though they generally have inferior efficacy for UTIs 3

Clinical Pearls

• Rising rates of trimethoprim-sulfamethoxazole resistance among uropathogens may limit its use as first-line therapy in some regions 3
• For severe infections requiring hospitalization, consider initial IV therapy before transitioning to oral treatment 3
• For recurrent infections, consider longer treatment courses or prophylactic regimens 1
• Single-dose therapy has been shown to be effective for uncomplicated UTIs in women but is not standard practice for most infections 5