What is the recommended dose of fosfomycin for a typical adult patient with a urinary tract infection (UTI) and normal renal function?

Fosfomycin Dosing for Urinary Tract Infections

For uncomplicated UTI in adult women with normal renal function, administer a single 3-gram oral dose of fosfomycin tromethamine. 1, 2, 3

Standard Dosing Regimen

Single-Dose Treatment (First-Line)

• The FDA-approved dose is one 3-gram sachet of fosfomycin tromethamine granules mixed with water, taken orally with or without food 3
• This single dose provides therapeutic urinary concentrations for 24-48 hours, sufficient to eradicate most uropathogens 1, 2
• Multiple major guidelines (IDSA, EAU, AUA, American College of Physicians) recommend this as first-line therapy for uncomplicated cystitis in women with A-I evidence rating 1

Alternative Multi-Dose Regimens (Off-Label)

For Complicated Lower UTI or Treatment Failures

• Administer 3 grams orally every 48-72 hours for a total of 3 doses in patients with complicated lower UTI, multidrug-resistant pathogens, or prior treatment failure 4, 5
• Pharmacokinetic modeling demonstrates that two 3-gram doses given 72 hours apart maintain urinary concentrations above the critical threshold for 161 hours (66% efficacy time over 7 days) 6

For Recurrent UTI Prevention

• Prophylactic dosing of 3 grams every 10 nights for 6 months has been studied, showing fewer infections per patient-year compared to placebo 7

Intravenous Dosing (Severe Infections)

For Complicated Upper UTI or Multidrug-Resistant Infections

• IV fosfomycin 6 grams every 8 hours for 7 days (14 days if concurrent bacteremia) is effective for complicated upper UTI and acute pyelonephritis 4
• For ICU patients with Acinetobacter baumannii infections and normal renal function, use 12-24 grams/day divided into 3-4 doses, always in combination therapy 7

Critical Limitations and Contraindications

Do NOT Use Fosfomycin For:

• Pyelonephritis or upper UTIs (oral formulation lacks sufficient efficacy data) 1, 2
• Uncomplicated UTI in men (limited efficacy data) 1
• Complicated UTIs as monotherapy (use multi-dose regimen or IV formulation) 1, 4

Use With Caution In:

• Patients with hypernatremia, cardiac insufficiency, or severe renal insufficiency (eGFR <30 mL/min/1.73 m²), as elimination half-life increases from 5.7 hours to 40-50 hours in anuric patients 1
• Monitor electrolytes during treatment, as fosfomycin can cause hypokalemia, hypocalcemia, hypomagnesemia, and hypernatremia 1

Renal Dosing Adjustments

• No dose adjustment needed for mild to moderate renal impairment (eGFR ≥30 mL/min/1.73 m²), including CKD stage 3b 1
• For severe renal impairment (eGFR <30 mL/min/1.73 m²), use with extreme caution and consider alternative agents 1

Clinical Advantages

• Single-dose convenience eliminates compliance issues associated with 3-7 day regimens 1, 2
• Minimal disruption to intestinal flora compared to fluoroquinolones and cephalosporins, reducing risk of C. difficile infection 1
• Effective against multidrug-resistant pathogens including ESBL-producing organisms, VRE, and MRSA 1, 2
• Low resistance rates: only 2.6% prevalence in initial E. coli infections 1

When Treatment Fails

• If symptoms do not resolve within 2-3 days or recur within 2 weeks, obtain urine culture and consider alternative agents: nitrofurantoin 100mg twice daily for 5 days, TMP-SMX 160/800mg twice daily for 3 days, or fluoroquinolones for 3 days 1
• Do not repeat fosfomycin for treatment failures; switch to a different antibiotic class 1

Special Populations

Pregnant Women:

• Fosfomycin is safe in pregnancy and recommended for asymptomatic bacteriuria as either standard short-course or single-dose treatment 1, 2

Community Resistance Patterns:

• Consider fosfomycin as first-line when TMP-SMX resistance exceeds 20-30% in your community 1