What is the recommended dosage of Bactrim (sulfamethoxazole and trimethoprim) for a teenage boy?

Bactrim Dosage for a Teenage Boy

For a teenage boy with a standard infection (such as urinary tract infection or acute otitis media), administer Bactrim based on weight: 8-10 mg/kg/day of trimethoprim (40-50 mg/kg/day sulfamethoxazole) divided into two doses every 12 hours, which typically translates to 2 DS tablets (or 4 single-strength tablets) every 12 hours for a teen weighing approximately 40 kg (88 lbs) or more. 1, 2

Weight-Based Dosing Algorithm

The dosing depends on the teenager's weight and infection severity:

For Mild-to-Moderate Infections (UTI, Skin Infections, Otitis Media)

• Weight 40 kg (88 lbs): 2 single-strength tablets (400/80 mg) or 1 DS tablet every 12 hours 3, 2
• Weight 30 kg (66 lbs): 1½ single-strength tablets or ¾ DS tablet every 12 hours 3, 2
• Weight 20 kg (44 lbs): 1 single-strength tablet or ½ DS tablet every 12 hours 3, 2

This achieves the recommended 8-10 mg/kg/day of trimethoprim component. 1

For Severe Infections (MRSA, Serious Bacterial Infections)

• Dose: 10-12 mg/kg/day of trimethoprim, up to 15-20 mg/kg/day divided every 6-8 hours for life-threatening infections 1
• This higher dosing is critical for achieving therapeutic targets against resistant organisms with MIC ≤0.5 mg/L 1

Formulation Selection

• Teenagers weighing >40 kg: Tablet formulation (DS tablets preferred for convenience) 3, 2
• Teenagers weighing <40 kg: Liquid formulation may provide more accurate dosing 1, 3

Duration of Treatment

• UTI/Acute otitis media: 10-14 days 2
• Shigellosis: 5 days 2
• Severe MRSA osteomyelitis: >6 weeks (typically combined with rifampin) 1

Critical Safety Monitoring

Before initiating therapy, obtain:

• Complete blood count with differential and platelet count 1, 3
• Renal function (creatinine clearance) 2
• Screen for G6PD deficiency risk 1

During prolonged therapy:

• Monthly CBC monitoring to detect hematologic toxicity (thrombocytopenia, neutropenia) 1, 3

Renal Impairment Adjustments

If the teenager has impaired renal function:

• CrCl 15-30 mL/min: Reduce dose to 50% of usual 3, 2
• CrCl <15 mL/min: Use not recommended 2

Important Drug Interactions and Precautions

• Methotrexate: Increased toxicity risk, but not contraindicated—monitor closely 3
• Warfarin: Enhanced anticoagulant effect—monitor INR 1
• Oral hypoglycemics: Increased hypoglycemia risk 1
• NSAIDs/salicylates: Can be given concurrently if renal function is normal 3

Common Pitfalls to Avoid

• Do not use test doses when initiating therapy—this is unnecessary and delays treatment 3
• Do not underdose based on adult tablets alone—weight-based calculations are essential for optimal outcomes 1, 2
• Do not forget to adjust for renal impairment—failure to do so increases toxicity risk significantly 2
• Avoid in patients with G6PD deficiency due to hemolytic anemia risk 1