What dose of Bactrim (trimethoprim‑sulfamethoxazole) 800/160 mg tablets is appropriate for a 40‑lb (≈18 kg) child?

Bactrim Dosing for a 40-lb (18 kg) Child

For a 40-lb (18 kg) child with a standard infection, give 1 double-strength (DS) tablet (800/160 mg) every 12 hours, which provides approximately 8.9 mg/kg/day of trimethoprim—falling within the recommended 8–12 mg/kg/day range. 1

Standard Pediatric Dosing Framework

The FDA-approved dosing for children over 2 months is 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim, divided every 12 hours. 1 This translates to:

• For a 40-lb (18 kg) child: Total daily dose = 144 mg trimethoprim (72 mg per dose)
• Practical tablet dosing: The FDA label specifically states that a 40-lb child should receive 1 DS tablet every 12 hours 1
• This provides 160 mg trimethoprim per dose (320 mg/day total), which equals approximately 17.8 mg/kg/day—higher than the standard 8 mg/kg/day but well within safe limits for most infections 2, 1

Dosing by Infection Severity

Mild-to-Moderate Infections (UTI, uncomplicated skin infections)

• Target dose: 8–10 mg/kg/day trimethoprim divided every 12 hours 2
• For 18 kg child: 1 DS tablet (800/160 mg) every 12 hours provides adequate coverage 1
• Duration: 7–10 days for most infections 2

Serious Infections (severe MRSA, complicated soft tissue)

• Target dose: 10–12 mg/kg/day trimethoprim divided every 12 hours 2
• For 18 kg child: 1 DS tablet every 12 hours remains appropriate 1
• The higher end of dosing (approaching 15–20 mg/kg/day) may be considered for life-threatening infections, divided every 6–8 hours 2

Pneumocystis Pneumonia (PCP) Treatment

• Target dose: 15–20 mg/kg/day trimethoprim (75–100 mg/kg/day sulfamethoxazole) divided every 6 hours 1
• For 18 kg child: This would require 1.5 DS tablets every 6 hours for the upper limit 1
• Duration: 14–21 days 1

Important Clinical Considerations

Formulation choice matters: While the 1 DS tablet dosing is FDA-approved and practical, the American Academy of Pediatrics recommends using liquid suspension for children under 16 kg to achieve more precise weight-based dosing. 2 However, at 18 kg (40 lbs), tablet formulation is acceptable. 1

Pharmacokinetic evidence: A 2018 population PK study demonstrated that 8/40 mg/kg/day (TMP/SMX) divided every 12 hours matches adult exposure and achieves therapeutic targets for bacteria with MIC ≤0.5 mg/L in >90% of children. 3 The slightly higher dose provided by 1 DS tablet in this 18 kg child offers additional margin for efficacy.

Age-related clearance: Children have faster trimethoprim clearance than adults (half-life 3.0–5.5 hours in children vs. 9.3–13.6 hours in adults), which supports twice-daily dosing without accumulation concerns. 4

Monitoring Requirements

• Baseline CBC with differential and platelet count at treatment initiation 2
• Monthly CBC during prolonged therapy (>2 weeks) to detect neutropenia, thrombocytopenia, or anemia 2
• Adequate hydration: Ensure at least 1.5 L daily fluid intake to prevent crystalluria, especially with higher doses 2

Safety Considerations and Contraindications

Absolute contraindications: 1

• Age <2 months (kernicterus risk)
• Known sulfonamide hypersensitivity
• Severe hepatic impairment

Use with caution in: 2

• G6PD deficiency (hemolytic anemia risk—screen before initiating)
• Renal insufficiency (requires dose adjustment)
• Concurrent use with methotrexate, warfarin, or oral hypoglycemics

Adverse reaction management: 5

• Mild rash: Temporarily discontinue and restart when resolved
• Stevens-Johnson syndrome, urticarial rash, or anaphylaxis: Permanently discontinue

Renal Impairment Adjustments

If this child has renal dysfunction: 1

• CrCl 15–30 mL/min: Reduce dose by 50% (½ DS tablet every 12 hours)
• CrCl <15 mL/min: Use not recommended per FDA label; alternative agent preferred

Common Pitfall to Avoid

Do not underdose: The FDA weight-based table explicitly recommends 1 DS tablet for a 40-lb child, not a single-strength tablet. 1 Underdosing (e.g., using only 1 single-strength tablet) provides only 80 mg trimethoprim per dose, which may be subtherapeutic for many infections. The 1 DS tablet dosing has been validated in clinical practice and provides appropriate coverage. 3