What is the recommended fosfomycin dosage for uncomplicated urinary tract infection in adults (women and men) and in pediatric patients?

Fosfomycin Dosage for Urinary Tract Infection

For uncomplicated cystitis in adult women, administer fosfomycin trometamol 3 grams as a single oral dose. This regimen achieves approximately 91% clinical cure rates and maintains therapeutic urinary concentrations for 24–48 hours. 1

Standard Adult Dosing

• Single 3-gram oral dose is the evidence-based regimen for uncomplicated lower urinary tract infections in women. 1
• This single dose provides sufficient urinary drug levels for 24–48 hours to eradicate most uropathogens. 1
• Clinical recovery rates of 88.9% and bacteriological eradication rates of 94.9% have been demonstrated at 8–10 days post-treatment. 2

Patient Population Restrictions

• Fosfomycin is recommended ONLY for uncomplicated cystitis in women—it should not be used for pyelonephritis, complicated UTIs, or routinely in men due to limited efficacy data for these populations. 1
• The European Urology guidelines explicitly state that fosfomycin lacks sufficient evidence for upper urinary tract infections and should be avoided when pyelonephritis is suspected. 1

Pregnancy-Specific Dosing

• The same single 3-gram oral dose is safe and effective during pregnancy, including the first trimester. 3, 4
• Fosfomycin is recommended by the European Association of Urology as first-line therapy (alongside nitrofurantoin) for both asymptomatic bacteriuria and symptomatic UTI in pregnant women. 3
• A single 3-gram dose shows similar bacteriological efficacy to 5-day cefuroxime or 7-day amoxicillin-clavulanate courses in pregnant women with asymptomatic bacteriuria. 4

Pediatric Dosing

• No pediatric dosing recommendations are provided in current guidelines; fosfomycin is not established as standard therapy for children with UTI. 1

Comparative Efficacy Context

• While fosfomycin's bacteriological eradication rates are somewhat lower than 3-day trimethoprim-sulfamethoxazole or fluoroquinolones, its overall clinical efficacy is comparable with the major advantage of single-dose convenience and minimal disruption to intestinal flora. 1
• A 2018 randomized trial found that 5-day nitrofurantoin achieved superior clinical resolution (70% vs 58%) and microbiologic resolution (74% vs 63%) compared to single-dose fosfomycin at 28 days. 5
• However, fosfomycin maintains extremely low resistance rates (2.6% in initial E. coli infections, 5.7% at 9 months), making it valuable when other first-line agents face high local resistance. 1

Position in Treatment Algorithm

• Use fosfomycin as first-line therapy when local E. coli resistance to trimethoprim-sulfamethoxazole exceeds 20%. 1
• Fosfomycin is particularly appropriate for multidrug-resistant pathogens including ESBL-producing E. coli, vancomycin-resistant Enterococcus, and MRSA causing uncomplicated cystitis. 1

Critical Contraindications

• Do NOT use fosfomycin for suspected pyelonephritis or upper urinary tract infections—insufficient tissue penetration renders it ineffective for these conditions. 1
• Switch to fluoroquinolones or parenteral cephalosporins if fever >38°C, flank pain, or costovertebral angle tenderness suggest upper tract involvement. 1

Renal Function Considerations

• Fosfomycin can be used at standard dosing (3 grams single dose) in patients with mild to moderate renal impairment (eGFR ≥30 mL/min/1.73 m²) without dose adjustment. 1
• For patients with severe renal dysfunction or anuric patients, elimination half-life increases from 5.7 hours to 40–50 hours; use with caution and monitor electrolytes. 1

Safety and Adverse Effects

• The most common adverse events are gastrointestinal: diarrhea, nausea, and vomiting occur in 4.3%–28% of patients. 1, 2
• Fosfomycin can cause hypokalemia, hypocalcemia, hypomagnesemia, and hypernatremia; monitor electrolytes in patients with pre-existing renal dysfunction, cardiac insufficiency, or hypernatremia. 1
• Patients with these conditions should use fosfomycin with caution, particularly when considering IV formulations for carbapenem-resistant organisms. 1

Management of Treatment Failure

• If symptoms do not resolve by the end of treatment or recur within 2 weeks, obtain urine culture and antimicrobial susceptibility testing. 1
• Switch to a different antibiotic class for a 7-day course—consider nitrofurantoin for 5 days, TMP-SMX for 3 days, or fluoroquinolones for 3 days based on culture results. 1

Off-Label Multi-Dose Regimen

• The European Urology guidelines suggest fosfomycin trometamol 3 grams on days 1,3, and 5 as an alternative regimen for gonococcal urethritis (off-label use). 1
• This multi-dose regimen is NOT recommended for standard uncomplicated UTI treatment. 1