Amoxicillin Dosing for Amoxicillin-Susceptible UTI
For uncomplicated UTIs caused by amoxicillin-susceptible organisms, amoxicillin is NOT recommended as first-line empirical therapy due to high global resistance rates (median 75% of E. coli isolates), but when susceptibility is confirmed, use 500 mg orally every 8 hours for 3-7 days in adults, or 20-40 mg/kg/day divided every 8-12 hours in children.
Critical Context: When Amoxicillin Should NOT Be Used
The 2024 WHO guidelines explicitly removed amoxicillin from recommended options for lower UTI treatment after 2020 GLASS data showed 75% median resistance rates (range 45-100%) in E. coli urinary isolates across 22 countries 1. Amoxicillin should never be used for empirical treatment of UTI 2. However, when culture confirms susceptibility, it remains a viable option.
Adult Dosing (When Susceptibility Confirmed)
Standard Dosing
- Mild to moderate genitourinary tract infections: 500 mg every 12 hours OR 250 mg every 8 hours 3
- Severe infections: 875 mg every 12 hours OR 500 mg every 8 hours 3
Duration
- Uncomplicated cystitis: 3-7 days based on clinical response 3
- Continue for minimum 48-72 hours beyond symptom resolution 3
Special Populations
Renal impairment 3:
- GFR 10-30 mL/min: 500 mg or 250 mg every 12 hours
- GFR <10 mL/min: 500 mg or 250 mg every 24 hours
- Hemodialysis: Additional dose during and after dialysis
Pediatric Dosing (≥3 Months Old)
Standard Dosing
For children <40 kg 4:
- Mild to moderate infections: 20-25 mg/kg/day divided every 8-12 hours
- Severe infections: 40-45 mg/kg/day divided every 8-12 hours
For children ≥40 kg: Use adult dosing 3
Infants <3 Months
- Maximum 30 mg/kg/day divided every 12 hours due to immature renal function 3
Duration
- Minimum 7-14 days for febrile UTI in children 4
- At least 10 days for any Streptococcus pyogenes infection 3
Alternative: Amoxicillin-Clavulanate (Preferred When Available)
Amoxicillin-clavulanate is superior to amoxicillin alone and is recommended as first-line therapy in multiple guidelines 1, 5:
Adult Dosing
- Standard: 500 mg amoxicillin component every 8 hours
- Alternative: 875 mg every 12 hours
Pediatric Dosing
Special Considerations for VRE UTI
For uncomplicated UTI caused by vancomycin-resistant enterococci (VRE) that are ampicillin/amoxicillin-resistant by standard testing, high-dose amoxicillin may still be effective 6:
- Amoxicillin 500 mg IV or PO every 8 hours 6
- Ampicillin 18-30 g/day IV in divided doses (alternative) 6
- Duration: 3-7 days 6
This paradoxical efficacy occurs despite in vitro resistance and has been demonstrated in clinical practice 7, with cure rates of 88% in complicated UTI.
Historical Context: Single-Dose Therapy
Single-dose amoxicillin 3 g was studied extensively in the 1980s for uncomplicated cystitis 8, 9, 10, 11:
- Cure rates: 60-90% depending on antibody-coated bacteria (ACB) status
- ACB-negative infections: 90% cure rate
- ACB-positive infections: 59% cure rate
This approach is no longer recommended given current resistance patterns and availability of superior alternatives.
Administration Pearls
- Take at the start of meals to minimize GI intolerance 3
- Oral suspension: Shake well before use; discard after 14 days 3
- Refrigeration preferred but not required for suspension 3
Common Pitfalls
- Never use empirically: Always confirm susceptibility first given 75% resistance rates 1
- Don't use for pyelonephritis: Amoxicillin alone does not achieve adequate tissue concentrations for upper tract infections 4
- Avoid in pregnancy without susceptibility data: Though historically used 12, current resistance rates make empiric use inappropriate
- Don't underdose children: Febrile UTI requires higher doses (40-45 mg/kg/day) 4