Amoxicillin Dosing for Uncomplicated UTI in Adults
Amoxicillin is NOT recommended as a first-line agent for uncomplicated UTI in adults due to high E. coli resistance rates, but if used for culture-proven susceptible organisms (particularly Enterococcus faecalis), the dose is 500 mg orally every 8 hours for 7 days. 1
Why Amoxicillin Should Be Avoided for Empiric UTI Treatment
- Plain amoxicillin should never be used for empirical UTI treatment due to high resistance rates worldwide 2
- The American College of Physicians acknowledges amoxicillin 500 mg every 8 hours for 7 days as a dosing option, but explicitly states it is not first-line due to poor E. coli susceptibility 1
- First-line agents for uncomplicated cystitis are nitrofurantoin, trimethoprim-sulfamethoxazole (if local resistance <20%), or fosfomycin 2
When Amoxicillin IS Appropriate
Amoxicillin 500 mg orally every 8 hours is first-line therapy specifically for uncomplicated pansensitive Enterococcus faecalis UTIs, achieving high clinical and microbiological eradication rates 1
Key Requirements Before Using Amoxicillin:
- Obtain culture and susceptibility testing before initiating amoxicillin for UTI 1
- Reserve amoxicillin for culture-proven susceptible organisms, particularly Enterococcus species 1
- Treatment duration is 7 days for susceptible infections 1
FDA-Approved Dosing for Genitourinary Tract Infections
According to the FDA label, for adults and pediatric patients ≥40 kg with genitourinary tract infections 3:
- Mild/Moderate 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
- Treatment should continue for a minimum of 48-72 hours beyond symptom resolution or evidence of bacterial eradication 3
Amoxicillin-Clavulanate as an Alternative
If considering the clavulanate combination (which has broader coverage):
- Dosing: 500 mg/125 mg every 8 hours OR 875 mg/125 mg every 12 hours for 3-7 days 2
- However, amoxicillin-clavulanate is also NOT first-line for uncomplicated cystitis, with inferior efficacy (58% cure rate at 4 months) compared to fluoroquinolones (77% cure rate) 4
- Beta-lactams have inferior efficacy compared to first-line agents and cause more adverse effects 2
Renal Dosing Adjustments
For patients with severe 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: 500 mg or 250 mg every 24 hours, with additional dose during and at end of dialysis
Critical Clinical Pitfalls
- Do not use amoxicillin empirically without culture results, as resistance rates make treatment failure likely 1, 2
- All male UTIs are considered complicated by definition and require 7-14 day treatment duration 4
- If using amoxicillin-clavulanate, monitor for treatment failure and switch to parenteral therapy or broader-spectrum agents if symptoms don't improve within 48-72 hours 4
- Take amoxicillin at the start of a meal to minimize gastrointestinal intolerance 3