Colistin is NOT indicated for the treatment of diarrhea
Colistin should never be used as a treatment for infectious diarrhea in clinical practice. This antibiotic is reserved exclusively for life-threatening infections caused by multidrug-resistant Gram-negative bacteria (particularly carbapenem-resistant organisms) and has no role in diarrheal illness management 1, 2.
Why Colistin Has No Role in Diarrhea Treatment
Guideline-Based Treatment for Infectious Diarrhea
The 2017 IDSA guidelines for infectious diarrhea provide clear recommendations that do not include colistin 1:
- For bloody diarrhea: Empiric therapy should be either a fluoroquinolone (such as ciprofloxacin) or azithromycin, depending on local susceptibility patterns and travel history 1
- For watery diarrhea: In most patients without recent international travel, empiric antimicrobial therapy is not recommended at all 1
- For traveler's diarrhea: Azithromycin is the preferred first-line agent (1 gram single dose or 500 mg daily for 3 days), particularly for dysentery or travel to Southeast Asia 3
Colistin's Actual Indications
Colistin is reserved for severe systemic infections caused by carbapenem-resistant organisms 1, 2:
- Approved use: Treatment of multidrug-resistant Acinetobacter baumannii and Pseudomonas aeruginosa infections in critically ill patients 1, 4
- Route of administration: Intravenous (as colistimethate sodium prodrug) or nebulized for pneumonia—never oral for systemic infections 1, 5
- Typical dosing: Loading dose of 9 million IU, followed by maintenance doses of 4.5 million IU every 12 hours for severe infections 1, 6
Critical Safety Concerns That Preclude Use in Diarrhea
Severe Nephrotoxicity Risk
Colistin causes significant kidney damage that would be completely unjustifiable for treating diarrhea 6:
- Nephrotoxicity incidence: Approximately 36% in critically ill patients, representing a 2.4-fold increased risk compared to β-lactam regimens 6
- Monitoring requirements: Renal function must be monitored 2-3 times per week during treatment 6
- Acute kidney injury: A major factor related to clinical failure and mortality during colistin therapy 6
Resistance Development Concerns
Using colistin inappropriately promotes resistance 7, 8:
- Colistin monotherapy rapidly selects resistant subpopulations, which is why only combination therapy is advised for its approved indications 8
- The plasmid-mediated mcr-1 colistin resistance gene has been identified in gut microbiota of diarrhea patients, demonstrating that inappropriate use could promote resistance transmission 7
The Correct Approach to Diarrhea Treatment
Severity-Based Algorithm
Follow this evidence-based approach instead 1, 3:
Mild diarrhea:
- Loperamide (4 mg loading dose, then 2 mg after each loose stool, maximum 16 mg/day) plus oral hydration 3
- No antibiotics needed 1
Moderate diarrhea:
- Azithromycin 500 mg daily for 3 days or single 1-gram dose 3
- Can combine with loperamide for faster symptom relief 3
Severe diarrhea or dysentery (fever with bloody stools):
- Azithromycin 1-gram single dose (mandatory first-line) 3
- Do NOT use loperamide if fever or blood in stool present 3
Special populations:
- Infants <3 months with suspected bacterial etiology: Third-generation cephalosporin 1, 3
- Pregnant women and children: Azithromycin preferred 3
Common Pitfall to Avoid
Never confuse oral colistin's role in selective digestive tract decontamination (SDD) in ICU patients with treatment of infectious diarrhea 8. Oral colistin in SDD protocols is used prophylactically to suppress gut colonization with resistant organisms in mechanically ventilated patients—this is completely different from treating active diarrheal illness and should never be extrapolated to that indication 8.