When should I prescribe antibiotics for acute diarrhea and which first‑line agents and dosing are appropriate for adults and children?

When to Prescribe Antibiotics for Acute Diarrhea

In most adults with acute watery diarrhea, do not prescribe antibiotics—rehydration is the priority; reserve empiric antibiotics for fever ≥38.5°C with bloody stools, recent international travel with severe symptoms, suspected Shigella dysentery, or immunocompromised patients with severe illness. 1, 2

Absolute Contraindications to Antibiotics

• Never prescribe antibiotics for suspected or confirmed Shiga toxin-producing E. coli (STEC O157:H7) because they markedly increase the risk of hemolytic uremic syndrome. 1, 2, 3
• Obtain Shiga toxin testing before starting antibiotics in any patient with bloody diarrhea. 2, 3
• Never treat asymptomatic household contacts of patients with diarrhea. 1, 2

When Antibiotics ARE Indicated

High-Risk Clinical Features Requiring Empiric Treatment

• Fever ≥38.5°C documented in a medical setting PLUS bloody or mucoid stools (presumptive Shigella, invasive E. coli, or Campylobacter). 1, 2, 3
• Bacillary dysentery syndrome: frequent scant bloody stools, high fever, severe abdominal cramps, and tenesmus. 1, 2
• Recent international travel with fever ≥38.5°C or signs of sepsis. 1, 2
• Immunocompromised patients (HIV, transplant, chemotherapy) with severe illness and bloody diarrhea. 1, 2
• Suspected enteric fever with sepsis features (altered mental status, hypotension, sustained high fever). 1, 2
• Ill-appearing infants <3 months of age with suspected bacterial etiology. 1, 2

When Antibiotics Are NOT Indicated

• Uncomplicated watery diarrhea without fever, blood, or recent travel—even if symptoms are bothersome. 1, 2, 3
• Persistent watery diarrhea lasting ≥14 days without inflammatory features. 1
• Mild symptoms in immunocompetent adults without high-risk features. 1, 2

First-Line Antibiotic Regimens

Adults

Azithromycin is the preferred first-line agent due to widespread fluoroquinolone resistance in Campylobacter (>90% in Southeast Asia and India). 2, 3, 4

• Acute watery diarrhea with high-risk features: azithromycin 500 mg single dose. 2, 3, 4
• Febrile dysentery or severe illness: azithromycin 1,000 mg single dose. 2, 3, 4, 5
• Alternative regimen: azithromycin 500 mg once daily for 3 days. 2, 4

Fluoroquinolones (second-line) only if azithromycin unavailable or local susceptibility favorable:

• Ciprofloxacin 750 mg single dose OR 500 mg twice daily for 3 days. 2, 4
• Levofloxacin 500 mg single dose OR 500 mg once daily for 3 days. 2, 4
• Do NOT use fluoroquinolones for Southeast Asia travel due to >90% Campylobacter resistance. 2, 3

Pediatric Patients

• Infants <3 months: ceftriaxone 50 mg/kg/day intramuscularly or intravenously (third-generation cephalosporin preferred). 2, 3
• Children ≥3 months: azithromycin 10 mg/kg once daily for 3 days (max 500 mg/day). 2, 3
• Avoid fluoroquinolones in children <6 years due to musculoskeletal adverse effects. 3

Adjunctive Symptomatic Therapy

Loperamide (After Adequate Rehydration)

• May be used in immunocompetent adults with watery diarrhea once rehydration is achieved. 1, 6, 3
• Dosing: 4 mg initially, then 2 mg after each loose stool (max 16 mg/24 hours). 6, 4
• Contraindicated if fever or bloody stools are present (risk of toxic megacolon). 1, 6, 3
• Never use in children <18 years. 1, 6
• When combined with azithromycin, loperamide reduces illness duration from 59 hours to approximately 1 hour. 3

Rehydration: The Cornerstone of All Diarrhea Management

Rehydration prevents morbidity and mortality—not antibiotics. 1, 6

Oral Rehydration Solution (ORS)

• Reduced-osmolarity ORS (65–70 mEq/L sodium, 75–90 mmol/L glucose) is first-line for mild-to-moderate dehydration. 1, 6
• Prescribe 2,200–4,000 mL/day total fluid intake, exceeding ongoing losses. 6
• Continue ORS until clinical dehydration resolves and diarrhea stops. 1, 6

Intravenous Fluids

• Switch to isotonic IV fluids (lactated Ringer's or normal saline) for severe dehydration (≥10% deficit), altered mental status, inability to tolerate oral intake, or shock. 1, 6
• Maintain IV fluids until pulse, perfusion, and mental status normalize, then transition to ORS. 1, 6

Diagnostic Testing (Selective)

Obtain stool studies only when results will change management: 1, 7

• Fever with bloody or mucoid stools. 1, 7
• Severe dehydration or systemic illness. 1, 7
• Immunosuppression. 1, 7
• Suspected outbreak or nosocomial infection. 1, 7
• Recent hospitalization or antibiotic exposure (evaluate for C. difficile). 1, 7

Stool panel should include: 1, 2

• Bacterial culture (Salmonella, Shigella, Campylobacter, Yersinia). 1, 2
• Shiga toxin testing (or gene detection) to identify STEC. 1, 2
• C. difficile toxin assay if recent healthcare or antibiotic exposure. 1, 2

Antibiotic Stewardship and Modification

• Modify or discontinue antibiotics once a specific pathogen is identified. 1, 2
• If no improvement within 48–72 hours, reassess for antimicrobial resistance, fluid/electrolyte disturbances, or non-infectious causes. 2, 6
• Do not treat non-typhoidal Salmonella routinely; reserve antibiotics for high-risk patients (age <6 months or >50 years, immunosuppressed, prosthetic devices, severe atherosclerosis). 2

Critical Pitfalls to Avoid

• Never prioritize antibiotics over rehydration—dehydration, not diarrhea, drives mortality. 1, 6
• Never start antibiotics for bloody diarrhea before ruling out STEC with Shiga toxin testing. 1, 2, 3
• Never use loperamide when fever or bloody stools are present. 1, 6, 3
• Never prescribe empiric antibiotics for uncomplicated watery diarrhea—this promotes antimicrobial resistance without clinical benefit. 1, 2, 3
• Never use fluoroquinolones for Southeast Asia travel due to >90% Campylobacter resistance. 2, 3

Dietary Management

• Resume normal, age-appropriate diet immediately after rehydration. 1, 6
• Start with small, light meals; avoid fatty, heavy, spicy foods and caffeine. 6