What is the recommended dosage of Bactrim (trimethoprim/sulfamethoxazole) for a 16-year-old male weighing 61.6kg?

Bactrim Dosing for a 16-Year-Old Male Weighing 61.6kg

For a 16-year-old male weighing 61.6kg, the standard Bactrim dose is one double-strength (DS) tablet (800mg sulfamethoxazole/160mg trimethoprim) every 12 hours for most common infections, or two single-strength tablets every 12 hours as an equivalent alternative. 1

Dosing Algorithm by Clinical Indication

For Urinary Tract Infections or Acute Bacterial Infections

• Standard adult dosing applies: 1 DS tablet every 12 hours for 10-14 days 1
• At 61.6kg, this patient exceeds the 40kg threshold where pediatric weight-based dosing transitions to adult fixed dosing 2, 1
• Alternative formulation: 2 single-strength tablets (400mg/80mg each) every 12 hours 1

For Pneumocystis Jiroveci Pneumonia (PCP) Treatment

• Higher dosing required: 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim, divided every 6 hours 1
• For this 61.6kg patient: approximately 2 DS tablets every 6 hours (upper limit dosing) for 14-21 days 1
• This indication requires significantly higher doses than standard bacterial infections 2

For PCP Prophylaxis

• Prophylactic dosing: 1 DS tablet daily 1
• Alternative regimen: 1 DS tablet three consecutive days per week 3, 4
• The Centers for Disease Control and Prevention recommends 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole for pediatric prophylaxis 4

Key Clinical Considerations

Age-Appropriate Dosing Transition

• This 16-year-old should receive adult dosing 2
• Guidelines define adult dosing as beginning at 15 years of age 2
• Children weighing more than 40kg should be dosed as adults 2
• At 61.6kg, this patient clearly meets both age and weight criteria for adult dosing 2, 1

Renal Function Assessment

• Dose adjustment required if creatinine clearance <30 mL/min: reduce to half the usual regimen 1
• If creatinine clearance <15 mL/min: Bactrim use is not recommended 1
• Standard dosing appropriate if renal function is normal 1

Duration by Indication

• Urinary tract infections: 10-14 days 1
• Shigellosis or traveler's diarrhea: 5 days 1
• Acute exacerbations of chronic bronchitis: 14 days 1
• PCP treatment: 14-21 days 1

Important Safety Monitoring

Baseline and Ongoing Monitoring

• Obtain baseline complete blood count with differential and platelet count before initiating therapy 3
• Monitor monthly for hematologic toxicity, particularly thrombocytopenia, anemia, and granulocytopenia 3, 4
• Hematologic reactions, though rare, can occur and typically develop within two weeks of therapy initiation 5

Common Adverse Effects to Anticipate

• Gastrointestinal disturbances (nausea, vomiting) and skin rash are most common 3, 5
• These side effects occur less frequently with trimethoprim-sulfamethoxazole compared to other sulfonamide combinations 6
• Permanently discontinue if life-threatening toxicity occurs 3

Critical Contraindications

• Contraindicated in patients <2 months of age 1
• Avoid use in pregnancy at term per the American College of Obstetricians and Gynecologists 4
• Use caution in patients with folate deficiency or those at risk for hematologic complications 5