Recommended Dosage of Bactrim (Trimethoprim/Sulfamethoxazole)
For adults, the standard dose of Bactrim is one double-strength tablet (containing 160 mg trimethoprim and 800 mg sulfamethoxazole) twice daily or two regular-strength tablets twice daily for most infections. 1
Adult Dosing
Oral Administration
- Urinary Tract Infections: 1 double-strength tablet (160 mg TMP/800 mg SMX) every 12 hours for 10-14 days 1
- Shigellosis: 1 double-strength tablet every 12 hours for 5 days 1
- Acute Bronchitis: 1 double-strength tablet every 12 hours for 14 days 1
- Traveler's Diarrhea: 1 double-strength tablet every 12 hours for 5 days 1
- PCP Prophylaxis: 1 double-strength tablet daily 1
- PCP Treatment: 75-100 mg/kg/day SMX and 15-20 mg/kg/day TMP divided every 6 hours for 14-21 days 1
Intravenous Administration
- For severe infections requiring IV therapy, dosing is typically based on the trimethoprim component at 8-12 mg/kg/day divided in either 4 doses (IV) or 2 doses (oral) 2
Pediatric Dosing
- Contraindicated in infants under 2 months of age 1
For Children ≥2 months:
- Urinary Tract Infections/Otitis Media: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided into two doses given every 12 hours for 10 days 1
- Shigellosis: Same dosage as above but for 5 days 1
- PCP Prophylaxis: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim divided into two doses given every 12 hours, 3 consecutive days per week (total daily dose should not exceed 1600 mg SMX and 320 mg TMP) 1
Weight-Based Pediatric Dosing Guide for UTI/Otitis Media 1:
- 10 kg (22 lb): ½ tablet every 12 hours
- 20 kg (44 lb): 1 tablet every 12 hours
- 30 kg (66 lb): 1½ tablets every 12 hours
- 40 kg (88 lb): 2 tablets or 1 DS tablet every 12 hours
Dosage Adjustments for Renal Impairment
Dosage adjustments are necessary for patients with impaired renal function 1, 3:
- Creatinine clearance >30 mL/min: Standard regimen
- Creatinine clearance 15-30 mL/min: Half the usual regimen
- Creatinine clearance <15 mL/min: Not recommended
Special Considerations
- For PCP prophylaxis in HIV patients, the CDC recommends one double-strength tablet daily for adults and adolescents 2
- For patients unable to tolerate TMP-SMX due to adverse reactions, temporary discontinuation with rechallenge after 2 weeks may be attempted 2
- For severe skin and soft tissue infections, particularly those caused by MRSA, 1-2 double-strength tablets twice daily may be used 2
Monitoring
- Complete blood counts with differential and platelet count should be performed at initiation of TMP-SMX prophylaxis and at monthly intervals to assess hematologic toxicity 2
- Elderly patients may require dose adjustment as they tend to have higher peak concentrations and reduced clearance of trimethoprim 4
Potential Adverse Effects
- Common side effects include gastrointestinal disturbances, hypersensitivity skin reactions, and rarely Stevens-Johnson syndrome, blood dyscrasias, and hepatic necrosis 2
- Thrombocytopenia risk increases with higher serum TMP levels and longer treatment duration 5
Always consider local antimicrobial resistance patterns when prescribing Bactrim, particularly for empiric therapy of skin and soft tissue infections where MRSA prevalence may vary by region.