What is the appropriate dosing of trimethoprim‑sulfamethoxazole (Bactrim) oral suspension for adults and children, including adjustments for renal impairment, contraindications, and recommended treatment durations?

Bactrim Suspension Dosing

Standard Pediatric Dosing (Children >2 months)

For most infections in children, administer 8–12 mg/kg/day of trimethoprim (40–60 mg/kg/day sulfamethoxazole) divided into 2 doses given every 12 hours. 1

Suspension Concentration

• The liquid formulation contains 40 mg trimethoprim per 5 mL 1
• Use liquid formulation for accurate dosing in children weighing <16 kg 1

Weight-Based Dosing Examples

For achieving 8 mg/kg trimethoprim per dose every 12 hours: 1

Weight	Dose per administration
10 kg (22 lbs)	1 single-strength tablet equivalent (80 mg TMP/400 mg SMX)
20 kg (44 lbs)	1 single-strength tablet equivalent
30 kg (66 lbs)	1½ single-strength tablets equivalent

Practical Calculation Example

For a 31 kg child receiving 10 mg/kg/day: 1

• Total daily dose = 310 mg trimethoprim
• Divided into 2 doses = 155 mg per dose
• Volume needed = 19.4 mL per dose (155 mg ÷ 40 mg per 5 mL)

Indication-Specific Dosing

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

• 8–10 mg/kg/day trimethoprim divided every 12 hours for 7–10 days 1
• For a 50 kg child: approximately 10–12 mL suspension every 12 hours 1

Serious Infections (severe MRSA, complicated soft tissue)

• 10–12 mg/kg/day trimethoprim divided every 12 hours 1
• For a 50 kg child: approximately 14–16 mL suspension every 12 hours 1

Life-Threatening Infections (severe pneumonia, CNS infections)

• 15–20 mg/kg/day trimethoprim divided every 6–8 hours (four times daily) 1
• This higher frequency is critical for achieving adequate CNS penetration 1

Pneumocystis Jiroveci Pneumonia (PCP) Treatment

• 15–20 mg/kg/day trimethoprim (75–100 mg/kg/day sulfamethoxazole) divided every 6 hours for 14–21 days 2, 3
• After acute pneumonitis resolves, may switch to oral using same total daily dose if no malabsorption or diarrhea present 2

PCP Prophylaxis (Pediatric)

• 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole divided twice daily, given 3 consecutive days per week 1
• Maximum daily dose: 320 mg trimethoprim/1600 mg sulfamethoxazole 1

Adult Dosing

Standard Infections

• 1 double-strength tablet (160 mg TMP/800 mg SMX) every 12 hours for 10–14 days 3

MRSA Skin and Soft Tissue Infections

• 1–2 double-strength tablets twice daily for 7–10 days 1, 4
• Use higher end (2 DS tablets) for more severe infections 5

PCP Prophylaxis (Adult)

• 1 double-strength tablet daily 3
• Alternative: 1 double-strength tablet three times weekly on consecutive days (Monday-Wednesday-Friday) provides equivalent protection with fewer side effects 5, 6

Renal Impairment Adjustments

Dose reduction is mandatory when creatinine clearance falls below 30 mL/min to prevent severe toxicity. 1

Treatment Dosing Adjustments

• CrCl 15–30 mL/min: Reduce dose by 50% 1, 3
• CrCl <15 mL/min: Reduce dose by 50% or use alternative agent 1, 3

PCP Treatment in Renal Impairment

• CrCl 10–50 mL/min: Give 3–5 mg/kg trimethoprim every 12 hours (instead of every 6–8 hours) 1
• CrCl <10 mL/min: Give 3–5 mg/kg trimethoprim every 24 hours 1

Prophylaxis in Renal Impairment

• CrCl 15–30 mL/min: Use half the standard prophylactic dose 1
• CrCl <15 mL/min: Use half dose or consider alternative agent 1

Hemodialysis

• 500 mg three times weekly after dialysis for prophylaxis 5

Critical Safety Considerations and Contraindications

Absolute Contraindications

• Children <2 months of age (risk of kernicterus) 3
• Pregnancy at term/third trimester (kernicterus risk) 4
• Nursing mothers 4
• Documented sulfa allergy 4
• G6PD deficiency (hemolytic anemia risk) 5

Severe Hepatic Impairment

• Avoid use in severe hepatic impairment 1

Drug Interactions Requiring Caution

• Methotrexate at treatment doses: Risk of severe bone marrow suppression; avoid concurrent use 5
• Warfarin: Enhanced anticoagulant effect; monitor INR closely 1
• Oral hypoglycemics: Increased hypoglycemia risk 1

Monitoring Requirements

Baseline and Ongoing Monitoring

• Obtain baseline CBC with differential and platelet count before initiating therapy 1
• Repeat CBC monthly during prolonged therapy to detect neutropenia, thrombocytopenia, or anemia 1, 5
• Monitor renal function and electrolytes regularly during high-dose therapy 1

Hydration

• Ensure at least 1.5 liters daily fluid intake to prevent crystalluria, especially with high-dose therapy 1

Management of Adverse Reactions

Mild Rash

• Temporarily discontinue and restart once rash resolves 2, 1
• Desensitization may be attempted if drug is essential 1

Life-Threatening Reactions

• Permanently discontinue for: Stevens-Johnson syndrome, urticarial rash, anaphylaxis, severe hypotension 2, 1
• Do not rechallenge after severe reactions 2

Common Adverse Effects

• Rash, pruritus, nausea, leukopenia, transaminase elevation occur frequently but are rarely life-threatening 2
• Adverse reaction frequency is lower in children (~15%) compared to adults 2

Treatment Duration by Indication

• Standard bacterial infections: 7–14 days 1
• UTI/shigellosis: 5–10 days 3
• Acute bronchitis exacerbation: 14 days 3
• PCP treatment: 14–21 days 2, 3
• MRSA osteomyelitis: >6 weeks (typically combined with rifampin) 1

Common Pitfalls to Avoid

• Failure to adjust for renal impairment with CrCl <30 mL/min markedly increases toxicity risk 1
• Using Bactrim alone for non-purulent cellulitis: Poor activity against beta-hemolytic streptococci; requires combination therapy or alternative agent 4
• Inadequate hydration during high-dose therapy increases crystalluria risk 1
• Using for mixed aerobic-anaerobic infections without anaerobic coverage: Lacks anaerobic activity; requires combination therapy 4