What are the dosing guidelines for Bactrim (trimethoprim‑sulfamethoxazole) in adults and children for common infections, PCP prophylaxis/treatment, and how should the dose be adjusted for renal impairment?

Bactrim Dosing Guidelines

For adults with normal renal function, use 1 double-strength (DS) tablet (160 mg TMP/800 mg SMX) every 12 hours for most common infections, while children over 2 months require 8 mg/kg/day of trimethoprim divided every 12 hours. 1

Standard Adult Dosing

Common Infections

• Urinary tract infections and shigellosis: 1 DS tablet every 12 hours for 10-14 days (UTI) or 5 days (shigellosis) 1
• Acute exacerbations of chronic bronchitis: 1 DS tablet every 12 hours for 14 days 1
• Traveler's diarrhea: 1 DS tablet every 12 hours for 5 days 1
• MRSA skin and soft tissue infections: 2 DS tablets (320 mg TMP/1600 mg SMX) every 12 hours for 7-10 days 2

Severe Infections

• PCP treatment: 15-20 mg/kg/day of trimethoprim (75-100 mg/kg/day sulfamethoxazole) divided every 6 hours for 14-21 days 1
  • For an 80 kg adult, this equals approximately 2-2.5 DS tablets every 6 hours 1
• MRSA CNS infections (meningitis, brain abscess): 5 mg/kg/dose every 8-12 hours of trimethoprim component 3
• MRSA osteomyelitis: 3.5-4.0 mg/kg/dose every 8-12 hours of trimethoprim, typically combined with rifampin for >6 weeks 3

Prophylaxis

• PCP prophylaxis: 1 DS tablet daily 1
• Alternative prophylaxis schedule: 1 DS tablet three times weekly on consecutive days 3

Pediatric Dosing (>2 months of age)

Standard Treatment Dosing

• Most infections: 8 mg/kg/day trimethoprim (40 mg/kg/day sulfamethoxazole) divided every 12 hours 1, 2
• Serious infections: 10-12 mg/kg/day trimethoprim divided every 12 hours 2
• Life-threatening infections: Up to 15-20 mg/kg/day trimethoprim divided every 6-8 hours 2

Weight-Based Dosing Table for Standard Treatment

Weight	Dose (every 12 hours)
10 kg (22 lbs)	½ tablet
20 kg (44 lbs)	1 tablet
30 kg (66 lbs)	1½ tablets
40 kg (88 lbs)	2 tablets or 1 DS tablet
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PCP Treatment in Children

• Dosing: 15-20 mg/kg/day trimethoprim (75-100 mg/kg/day sulfamethoxazole) divided every 6 hours for 14-21 days 1

PCP Prophylaxis in Children

• Standard regimen: 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole, divided twice daily, given 3 consecutive days per week (maximum 320 mg TMP/1600 mg SMX daily) 1, 3, 4
• Alternative daily dosing: 8 mg/kg/day trimethoprim divided into two doses 4

HIV-Exposed/Infected Infants

• Initiation: Start prophylaxis at 4-6 weeks of age for all HIV-exposed infants 4
• Continue throughout first year until HIV infection definitively ruled out 4
• HIV-infected children: Continue based on age-specific CD4+ thresholds 4

Renal Impairment Adjustments

Prophylaxis Dosing

• CrCl 15-30 mL/min: Reduce dose by 50% (½ of standard dose) 3
• CrCl <15 mL/min: Reduce dose by 50% or use alternative agent 3

Treatment Dosing for PCP

• CrCl 10-50 mL/min: 3-5 mg/kg trimethoprim every 12 hours (instead of every 6-8 hours) 3, 2
• CrCl <10 mL/min: 3-5 mg/kg trimethoprim every 24 hours 3, 2

FDA-Approved Renal Dosing

• CrCl >30 mL/min: Use standard regimen 1
• CrCl 15-30 mL/min: Use ½ the usual regimen 1
• CrCl <15 mL/min: Use not recommended per FDA label, though clinical guidelines support reduced dosing 1, 3

Important consideration: In critically ill patients on continuous renal replacement therapy (CVVH), standard renal dosing may result in sub-therapeutic levels; higher doses (up to 640/3,200 mg three times daily) may be required with therapeutic drug monitoring 5

Special Populations and Monitoring

Hemodialysis Patients

• Prophylaxis: Low-dose regimens (<6 single-strength tablets/week) are effective and better tolerated than standard dosing 6
• Treatment: Dosing interval should be increased to 12 times the serum creatinine level in mg/dL (maximum 48 hours) 7

Monitoring Requirements

• Baseline: Complete blood count with differential and platelet count 2
• During therapy: Monthly CBC for prolonged therapy (>1 month) 2, 8
• Renal function: Monitor creatinine clearance and electrolytes, especially during high-dose therapy 3
• G6PD screening: Perform before initiating therapy due to hemolytic anemia risk 2

Hydration

• Maintain adequate fluid intake of at least 1.5 liters daily to prevent crystalluria, particularly important with high-dose therapy 3, 2

Important Drug Interactions and Contraindications

Contraindications

• Age <2 months: Not recommended 1
• Severe hepatic impairment: Avoid use 2
• G6PD deficiency: Use with extreme caution due to hemolytic anemia risk 2

Significant Drug Interactions

• Methotrexate: Increased toxicity risk 2
• Warfarin and other anticoagulants: Enhanced anticoagulant effect 2
• Oral hypoglycemics: Increased hypoglycemia risk 2

Formulation Considerations

Pediatric Patients

• Use liquid formulation for children weighing <16 kg to ensure accurate dosing 2
• Liquid suspension: Contains 40 mg TMP/200 mg SMX per 5 mL 8

Tablet Strengths

• Single-strength (SS): 80 mg TMP/400 mg SMX 1
• Double-strength (DS): 160 mg TMP/800 mg SMX 1

Common Pitfalls to Avoid

• Inadequate treatment duration: Using <7 days for febrile UTIs or pyelonephritis leads to treatment failure 8
• Inappropriate renal dose reduction: Failing to adjust for CrCl <30 mL/min increases toxicity risk 3, 1
• Nitrofurantoin substitution for febrile UTI: Does not achieve adequate tissue concentrations for pyelonephritis 8
• Ignoring local resistance patterns: Always consider antimicrobial susceptibility when selecting empiric therapy 8
• Inadequate monitoring: Failure to perform monthly CBCs during prolonged therapy may miss hematologic toxicity 2
• Insufficient hydration: Increases crystalluria risk, especially with high-dose therapy 3