What is the recommended dosage of Fosfomycin for treating uncomplicated urinary tract infections (UTIs)?

Fosfomycin Dosing for Uncomplicated UTIs

For uncomplicated urinary tract infections (acute cystitis) in women, the recommended dosage is a single 3-gram oral dose of fosfomycin tromethamine, mixed with water and taken once. 1, 2

Standard Dosing Regimen

• Single 3-gram oral dose is the FDA-approved and guideline-recommended treatment for uncomplicated cystitis in women aged 18 years and older 1, 3, 2
• The medication must be mixed with water before ingestion and should never be taken in dry form 1
• Can be taken with or without food 1
• This single dose provides therapeutic urinary concentrations for 24-48 hours 2

Clinical Efficacy

• Clinical cure rates of 88.9% and bacteriological eradication rates of 94.9% have been demonstrated with single-dose therapy 4
• The Infectious Diseases Society of America (IDSA) and European Society for Microbiology and Infectious Diseases classify fosfomycin as appropriate for uncomplicated cystitis, though it may have slightly inferior efficacy compared to 3-day trimethoprim-sulfamethoxazole or fluoroquinolones 3
• The American Urological Association recommends fosfomycin as one of three first-line therapies for uncomplicated UTIs (Grade B evidence) 2

Key Advantages

• Minimal collateral damage to intestinal flora compared to other antibiotics 2, 3
• Single-dose convenience improves adherence compared to 3-7 day regimens 2
• Low resistance rates among common uropathogens 3, 2
• Safe in pregnancy for both uncomplicated UTIs and asymptomatic bacteriuria 2, 5

Special Populations

Vancomycin-Resistant Enterococcus (VRE)

• The same single 3-gram oral dose is recommended for uncomplicated UTIs caused by VRE 3, 2

Pregnant Women

• Single-dose fosfomycin is recommended for asymptomatic bacteriuria in pregnancy, with efficacy comparable to 5-7 day courses of other agents 2, 5

Critical Limitations and Pitfalls

Do NOT use fosfomycin for:

• Pyelonephritis - insufficient efficacy data 2, 6
• Complicated UTIs - restricted to uncomplicated cystitis only 2, 6
• Men with UTIs - limited clinical efficacy data in this population 2
• Non-fermenting gram-negative organisms (e.g., Pseudomonas, Acinetobacter) - lacks sufficient data and poor efficacy 6

Adverse Effects

• Gastrointestinal symptoms (diarrhea, nausea, vomiting) are most common 2, 4
• Adverse events occur in approximately 4.3% of patients, typically mild 4
• Higher initial adverse event rates (43% at day 4) compared to nitrofurantoin (25%), though rates equalize by day 9 7

Follow-Up Considerations

• Routine post-treatment cultures are NOT indicated for asymptomatic patients 2
• If symptoms persist at end of treatment or recur within 2 weeks, perform urine culture and antimicrobial susceptibility testing 2