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Phosphonic Acid Antibiotic (Uncomplicated UTI / MDR UTI) Pregnancy: Oral 3 g single dose: safe in pregnancy — first-line for uncomplicated UTI in pregnant women (NICE 2022; preferred over nitrofurantoin from 36 weeks and trimethoprim in first trimester). IV formulation: use under specialist guidance only.

Fosfomycin

Brand names: Monurol (oral granules), Fomicyt (IV)

Adult dose

Dose: Oral (uncomplicated UTI): 3 g as single dose (dissolved in water). IV (complex/MDR): 16–24 g/day in 3–4 divided doses
Route: Oral (granules dissolved in water) or Intravenous
Frequency: Single dose (oral/uncomplicated); every 6–8 hours (IV/complex)
Max: Oral: 3 g single dose; IV: 24 g/day
Oral single-dose fosfomycin for uncomplicated lower UTI in women (E. coli, Enterococcus faecalis). IV fosfomycin for MDR UTI/complicated UTI (ESBL-producing, MRSA UTI). Excellent urinary concentration — urinary levels far exceed MIC of most uropathogens. Source: BNF 90; NICE 2022 UTI guidelines; EAU Guidelines.

Paediatric dose

Dose: Children >12 years: 3 g single dose oral (same as adult). Under 12 years: specialist guidance — not standard g/kg
Route: Oral
Frequency: Single dose
Max: 3 g
Oral fosfomycin licensed in UK from 12 years for UTI. IV formulation use in children under specialist ID guidance. Source: BNF for Children 2024.

Dose adjustments

Renal

Oral 3 g single dose: no adjustment for UTI (urinary concentration mechanism). IV: eGFR 40–80 mL/min: 12 g/day. eGFR 20–40 mL/min: 8 g/day. eGFR <20 mL/min: 4 g/day. Haemodialysis: 4 g post-dialysis.

Hepatic

No dose adjustment required — not hepatically metabolised.

Paediatric weight-based calculator

Oral fosfomycin licensed in UK from 12 years for UTI. IV formulation use in children under specialist ID guidance. Source: BNF for Children 2024.

Clinical pearls

  • Single-dose convenience — the primary appeal for uncomplicated UTI: one 3 g sachet dissolved in water, taken once. Equivalent clinical cure rates to nitrofurantoin 7-day course (77–80% clinical success) for uncomplicated cystitis in women. High adherence — no daily dosing. NICE 2022 UTI guidelines include fosfomycin as first-line alternative to nitrofurantoin or trimethoprim.
  • MDR UTI advantage — unique mechanism: fosfomycin inhibits the first step in bacterial cell wall synthesis (MurA enzyme — unique target not shared with any other antibiotic class). Therefore, no cross-resistance with beta-lactams, fluoroquinolones, aminoglycosides, or carbapenems. ESBL-producing E. coli, Klebsiella, and even some MRSA (UTI) are susceptible. IV fosfomycin used for MDR UTI when other options are exhausted — specialist ID input required.
  • IV sodium burden — critical monitoring: IV fosfomycin disodium contains 14–24 mmol sodium per gram of fosfomycin. At 16–24 g/day IV, patients receive 224–336 mmol Na+ per day — severely sodium-loading in cardiac and renal patients. Monitor serum sodium, daily weight, and fluid balance meticulously. May require furosemide.
  • NOT for pyelonephritis (oral route): oral fosfomycin achieves excellent urinary bladder concentrations but inadequate renal parenchymal concentrations for upper UTI. Use IV formulation or alternative antibiotics for pyelonephritis. Oral fosfomycin failure in pyelonephritis due to inadequate tissue penetration.
  • Resistance emergence with IV courses: fosfomycin single-point resistance mutations (murA overexpression) emerge readily in vitro and with prolonged monotherapy. Always combine IV fosfomycin with a second agent for serious infections. Source: BNF 90; NICE 2022 Urinary Tract Infections Guidelines; EAU UTI Guidelines 2023; MHRA SPC Monurol/Fomicyt.

Contraindications

  • Oral fosfomycin for pyelonephritis or complicated UTI (not adequately concentrated in renal parenchyma — only uncomplicated cystitis indication)
  • Sodium-restricted patients (oral formulation contains 48 mmol Na+ per sachet — equivalent to 3 g NaCl)
  • Hypersensitivity to fosfomycin

Side effects

  • Diarrhoea, nausea, headache (most common with oral — mild)
  • Vaginitis (GI flora alteration — rare with single dose)
  • IV: hypernatraemia, hypokalaemia (large sodium load — fosfomycin disodium; IV doses contain very high sodium)
  • IV: elevated liver transaminases (monitor LFTs during prolonged IV)

Interactions

  • Metoclopramide: reduces fosfomycin oral bioavailability by increasing GI motility — avoid or separate by 2 hours
  • Antacids: may reduce oral absorption — take fosfomycin with water on empty stomach
  • IV potassium depletion: large IV doses cause kaliuresis — proactively supplement potassium

Monitoring

  • Urine culture and susceptibility testing before prescribing (confirm fosfomycin MIC — usually ≤64 mg/L is susceptible for UTI)
  • Urine culture at 5–7 days and test of cure at 2 weeks (uncomplicated UTI)
  • IV: serum sodium daily (hypernatraemia from sodium load)
  • IV: serum potassium daily (hypokalaemia — supplement proactively)
  • IV: LFTs weekly
  • IV: fluid balance daily

Reference: BNFc; BNF 90; BNF for Children 2024; NICE 2022 Urinary Tract Infections Guidelines; EAU UTI Guidelines 2023; MHRA SPC Fomicyt/Monurol. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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