What is the dosage of Bactrim DS (trimethoprim/sulfamethoxazole) for treating a stool infection?

Bactrim DS Dosage for Stool Infections (Bacterial Gastroenteritis)

For bacterial stool infections in adults, the standard dose of Bactrim DS (trimethoprim-sulfamethoxazole 160/800 mg) is one double-strength tablet twice daily for 5 days. 1

Standard Dosing Regimen

Adults

• Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole): 1 double-strength tablet orally every 12 hours for 5 days 1
• This dosing is FDA-approved specifically for shigellosis and has proven efficacy for traveler's diarrhea and other bacterial gastroenteritis 1, 2

Pediatric Patients (≥2 months of age)

• 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided into two doses every 12 hours for 5 days 1
• Weight-based dosing table from FDA label 1:
  • 22 lb (10 kg): 1 tablet (400/80 mg) every 12 hours
  • 44 lb (20 kg): 2 tablets (400/80 mg) or 1 DS tablet every 12 hours
  • 66 lb (30 kg): 1.5 DS tablets every 12 hours
  • 88 lb (40 kg): 2 DS tablets every 12 hours

Clinical Evidence Supporting This Dosing

The 5-day regimen is highly effective for bacterial gastroenteritis:

• TMP-SMX achieved clinical resolution in an average of 1.7 days for shigellosis, with stool cultures becoming negative within 1.6 days 3
• In traveler's diarrhea, TMP-SMX reduced treatment failures to only 5% compared to 49% with placebo 2
• For Mexican children with bacterial diarrhea, TMP-SMX significantly shortened illness duration, particularly in those with fever or fecal leukocytes 4

Specific Clinical Scenarios

Shigellosis

• Same 5-day regimen as above 1, 3
• TMP-SMX is particularly valuable when ampicillin resistance is present, as it remains effective against resistant strains 3

Traveler's Diarrhea

• Standard dose: 1 DS tablet every 12 hours for 5 days 1
• Alternative single-dose therapy: 2 DS tablets (1600/320 mg) as a one-time dose has shown efficacy, reducing diarrhea duration from 59 hours to 28 hours 5
• Combination with loperamide (4 mg loading dose, then 2 mg after each loose stool) further reduces duration to approximately 1 hour, but should only be used when bacterial dysentery (bloody diarrhea) is excluded 5

Enterotoxigenic E. coli and Other Bacterial Pathogens

• Same 5-day regimen 1, 4
• Most effective when clinical signs suggest bacterial infection (fever, fecal leukocytes, or culture-proven bacterial pathogen) 4

Dose Adjustments for Renal Impairment

Renal function significantly affects dosing 1:

• CrCl >30 mL/min: Standard dosing
• CrCl 15-30 mL/min: Reduce dose by 50% (½ DS tablet every 12 hours)
• CrCl <15 mL/min: Use not recommended 1

Important Clinical Caveats

When NOT to Use Bactrim

• Contraindicated in infants <2 months of age 1
• Avoid in third trimester pregnancy 6
• Do not use empirically if local resistance rates exceed 20% 6
• Consider alternative agents if patient has sulfa allergy or history of severe reactions 7

Common Pitfalls

• Inadequate treatment duration: The full 5-day course should be completed even if symptoms resolve earlier to prevent relapse and resistance 1, 3
• Using for viral gastroenteritis: TMP-SMX is only effective for bacterial pathogens; it provides no benefit for viral or non-bacterial causes 4
• Ignoring resistance patterns: In areas with high TMP-SMX resistance among enteric pathogens, alternative antibiotics should be considered 6

Monitoring and Expected Response

• Clinical improvement should occur within 24-48 hours 2, 4
• Diarrhea typically resolves within 1.7-2.9 days of starting therapy 3, 5
• If no improvement by 48-72 hours, consider resistant organism or alternative diagnosis 8

Common Adverse Effects

• Rash, urticaria, nausea, vomiting, and gastrointestinal disturbances are most common 6
• Rare but serious: Stevens-Johnson syndrome, hematologic abnormalities 7, 6

Alternative Agents if Bactrim Cannot Be Used

If TMP-SMX is contraindicated or ineffective 7:

• Fluoroquinolones: Ciprofloxacin 500 mg orally every 12 hours for 3-5 days
• Azithromycin: 500 mg once daily for 3 days (particularly useful for Campylobacter)
• These alternatives should be reserved when TMP-SMX resistance is documented or suspected