Bactrim DS (Trimethoprim-Sulfamethoxazole Double-Strength): Clinical Guide
Formulation
Bactrim DS contains 160 mg trimethoprim + 800 mg sulfamethoxazole per tablet, while single-strength tablets contain half these amounts (80 mg/400 mg). 1
Standard Adult Dosing
For most bacterial infections in adults, prescribe 1 DS tablet twice daily. 1 This delivers 160 mg trimethoprim + 800 mg sulfamethoxazole per dose and provides adequate serum concentrations across infection types. 1
Indication-Specific Regimens
Urinary Tract Infections (Pyelonephritis)
- 1 DS tablet twice daily for 14 days when the pathogen is confirmed susceptible. 1
- Single-dose therapy (2 DS tablets once) achieves 93% cure rates for uncomplicated lower UTI in women but is associated with higher early recurrence (24% vs 5% at 2 weeks). 2, 3
MRSA Skin and Soft-Tissue Infections
- 1–2 DS tablets twice daily for 7 days, using 2 tablets for severe disease. 4, 1, 5
- Critical caveat: Never use Bactrim as monotherapy for non-purulent cellulitis because it lacks activity against beta-hemolytic streptococci. 1, 5 For mixed infections, combine with an agent covering streptococci. 5
Pneumocystis jirovecii Pneumonia (PCP)
- Prophylaxis: 1 DS tablet daily when CD4⁺ count <200 cells/µL in HIV-infected adults. 1
- Alternative prophylaxis: 1 DS tablet three times weekly on consecutive days provides equivalent protection with fewer adverse effects (40–65% experience reactions with daily dosing). 1, 5
- Treatment: 15–20 mg/kg/day trimethoprim (75–100 mg/kg/day sulfamethoxazole) divided every 6 hours IV for 14–21 days, then switch to oral at the same dose after acute pneumonitis resolves. 6
Severe MRSA Infections
- CNS infections/brain abscess: 5 mg/kg trimethoprim IV every 8–12 hours. 6, 5
- Osteomyelitis: 3.5–4 mg/kg trimethoprim every 8–12 hours, combined with rifampin for >6 weeks. 6
Pediatric Dosing
Standard dosing: 8–12 mg/kg/day trimethoprim (40–60 mg/kg/day sulfamethoxazole) divided every 12 hours. 6 This matches adult exposure and achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children. 1
Age-Specific Considerations
- Initiate prophylaxis at 4–6 weeks of age in HIV-exposed infants, continuing through the first year. 1
- Do not use in neonates or infants <2 months old due to kernicterus risk. 4
- Avoid in children <8 years when alternatives exist, though exceptional circumstances may warrant use. 4
Severity-Based Pediatric Dosing
- Mild-to-moderate infections: 8–10 mg/kg/day trimethoprim divided every 12 hours for 7–10 days. 6
- Serious infections (severe MRSA): 10–12 mg/kg/day trimethoprim divided every 12 hours. 6
- Life-threatening infections: 15–20 mg/kg/day trimethoprim divided every 6–8 hours (four times daily). 6
Practical Pediatric Dosing Example
For a 31 kg child requiring 10 mg/kg/day: total daily dose = 310 mg trimethoprim, divided into 155 mg per dose twice daily. 6 Using the suspension (40 mg trimethoprim per 5 mL), this equals approximately 19.4 mL per dose. 6
Pediatric PCP Prophylaxis
150 mg/m²/day trimethoprim + 750 mg/m²/day sulfamethoxazole, divided twice daily, given 3 consecutive days per week (maximum 320 mg trimethoprim/1600 mg sulfamethoxazole daily). 4, 6
Weight-based prophylaxis dosing: 6
- 10 kg: 1 single-strength tablet twice daily
- 20 kg: 1 single-strength tablet twice daily
- 30 kg: 1½ single-strength tablets twice daily
Renal Dose Adjustments
For CrCl 15–30 mL/min: reduce the dose by 50% (use single-strength tablets or half a DS tablet). 1, 6
For CrCl <15 mL/min: 6
- Prophylaxis: reduce by 50% or use an alternative agent
- Treatment: 3–5 mg/kg trimethoprim every 24 hours
For CrCl 10–50 mL/min during treatment: 3–5 mg/kg trimethoprim every 12 hours (instead of every 6–8 hours). 6
Critical warning: Failure to adjust dosing for CrCl <30 mL/min markedly increases toxicity risk. 6 Monitor creatinine clearance and electrolytes regularly during high-dose therapy. 6
Contraindications and Precautions
Absolute Contraindications
- Third trimester of pregnancy (kernicterus risk in the newborn). 5
- Nursing mothers (kernicterus risk). 5
- Documented sulfonamide allergy. 5
- Severe hepatic impairment. 6
High-Risk Populations Requiring Caution
- G6PD deficiency: Screen before initiating therapy due to hemolytic anemia risk. 1, 5
- Elderly patients: Advanced age independently increases acute kidney injury risk; monitor renal function closely. 5
- Patients on warfarin: Bactrim enhances anticoagulant effect; monitor INR. 6
- Patients on methotrexate: Bactrim increases methotrexate toxicity. 6
- Patients on oral hypoglycemics: Increased hypoglycemia risk. 6
Monitoring Requirements
Obtain complete blood count with differential and platelet count at initiation, then monthly during prolonged therapy to detect neutropenia, thrombocytopenia, and anemia. 4, 1, 6
Ensure adequate hydration (≥1.5 L daily) to prevent crystalluria, especially during high-dose therapy. 6
Monitor renal function and liver enzymes during prolonged treatment to detect toxicity early. 1
Adverse Effects Management
Common Reactions (24–38% of patients)
- Pruritus/rash
- Nausea and gastrointestinal upset
- Leukopenia
- Transient transaminase elevations 1
Management Algorithm
Mild rash: Temporarily discontinue, restart after resolution; desensitization may be attempted if the drug is essential. 6
Life-threatening reactions (anaphylaxis, Stevens-Johnson syndrome, severe hypotension): Permanently discontinue and never rechallenge. 4, 6
Pediatric note: Adverse reaction incidence is lower in children (approximately 15%) compared with adults. 6
IV-to-Oral Transition
Severe infections requiring parenteral therapy: 8–12 mg/kg/day trimethoprim IV divided into four doses, each infused over 1 hour. 5
Switch to oral therapy using the same total daily dose after clinical resolution of acute symptoms, provided the patient has no malabsorption or diarrhea. 6, 5
Cross-Protection Benefits
When used for PCP prophylaxis, Bactrim also protects against toxoplasmosis and a broad range of bacterial infections. 1