Fosfomycin
cis-1, 2-epoxyphosphonic acid
Also known as: Monurol · MK-955 · phosphomycin · phosphonomycin · fosfomicina · fosfomycine · fosfomycinum · fosfomysiini · fosfomycin trometamol
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Urinary Tract Infection (MDR E. coli) | 40 mg/kg | PO | Single dose | Single dose | 🌎 NA |
- Urinary Tract Infection (MDR E. coli): Investigational. Dose based on disodium salt in a PK study. Use should be prudently reserved for MDR E. coli based on susceptibility testing only when no other alternative exists.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections (Investigational) | 20 mg/kg | SC | q10h | Unknown | 🌎 NA |
- Susceptible infections (Investigational): Based on PK study using fosfomycin disodium (not commercially available in USA). May maintain effective plasma concentrations for up to 10 hours.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Fosfomycin is a unique, synthetic, broad-spectrum phosphonic acid derivative antibiotic. Originally isolated from Streptomyces fradiae, it is primarily utilized in human medicine as a single-dose treatment for uncomplicated urinary tract infections (UTIs).
In veterinary medicine, its use is highly investigational but gaining attention as a "rescue" or reserve antimicrobial for:
- Multi-drug resistant (MDR) Escherichia coli
- Extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae
- Refractory Enterococcus infections
Clinical Pearls:
- It is primarily a time-dependent bactericidal agent, though it exhibits some concentration-dependent characteristics.
- Cross-resistance with beta-lactams or aminoglycosides does not typically occur, making it a valuable option when standard first-line antibiotics fail.
- Important: The oral formulation available in the US is the tromethamine salt, while many veterinary pharmacokinetic studies have utilized the disodium salt (which is administered parenterally).
Mechanism of action
Fosfomycin exerts its bactericidal effect by interfering with the very first step of bacterial cell wall (peptidoglycan) synthesis.
- It acts as a structural analogue of phosphoenolpyruvate (PEP).
- It irreversibly binds to and inhibits the cytosolic enzyme MurA (UDP-N-acetylglucosamine enolpyruvyl transferase).
- Pathway: Blocks addition of PEP to UDP-N-acetylglucosamine → Prevents formation of uridine diphosphate-N-acetylmuramic acid (UDP-MurNAc) → Halts peptidoglycan synthesis → Bacterial cell lysis.
- Secondary Action: It also reduces the adherence of bacteria to uroepithelial cells, which is highly beneficial in treating UTIs.
- Resistance Mechanism: When resistance occurs, it is typically chromosomally mediated via the production of FosA or FosX enzymes, which hydrolyze and inactivate the drug.
Safety & warnings
Contraindications
- Young cats (Contraindicated due to nephrotoxicity)
- Adult cats (Relative contraindication; use with extreme caution if at all)
Adverse effects
- Diarrhea (most common in humans)
- Gastrointestinal upset
- Renal tubular damage / Nephrotoxicity (observed in cats)
Precautions
Warning: Appears to be highly nephrotoxic in cats, causing tubular necrosis and significant increases in serum creatinine. Use is generally contraindicated in felines.
- Renal Impairment: In humans, renal dysfunction can substantially increase the drug's half-life and reduce urine levels.
- Dosing Strategy: In humans, this drug is used as a one-time dose; multiple doses do not enhance efficacy and increase the incidence of adverse events. Veterinary protocols are still investigational.
Drug interactions
Can decrease serum concentrations and reduce urine levels of fosfomycin.
Theoretical risk of decreasing serum and urine concentrations by increasing GI motility, similar to metoclopramide.
Monitoring
- Gastrointestinal adverse effects (diarrhea)
- Clinical signs of UTI resolution
- Urine culture and susceptibility (pre- and post-treatment)
- Urinalysis
- Renal function tests (BUN, Creatinine) especially if used off-label in cats
Pharmacokinetics
Half-life
Absorption
Fosfomycin tromethamine is rapidly converted to the free acid fosfomycin after absorption. Dogs (PO): Bioavailability ~29%, peak levels at 2 hours. Horses (SC disodium salt): Bioavailability ~85%, peak at 3.25 hours.
Distribution
Distributes well into the kidneys, bladder wall, and prostate. Crosses the placenta. Volume of distribution (steady-state): ~0.7 L/kg (dogs), 0.21 L/kg (horses). Protein binding is very low.
Metabolism
Fosfomycin tromethamine is rapidly converted to the active free acid fosfomycin.
Elimination
Primarily eliminated as unchanged drug in the urine (38% of an oral dose in humans). Clearance: ~15 mL/kg/hr (dogs), 16-24 mL/kg/hr (horses).
Overdose
Single overdoses are most likely to cause gastrointestinal effects (e.g., severe diarrhea). In cats, overdosage or even standard doses can cause severe nephrotoxicity (tubular necrosis). Treatment should be supportive and symptomatic.
Available products
Formulations
- Oral granules (3 grams per packet)
Human-labeled
- Fosfomycin Tromethamine 3 grams per packet (for dilution and oral use) (Monurol)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Fosfomycin tromethamine granules should be stored at room temperature (25°C).
Client information
Important: This medication is typically reserved for highly resistant infections where other antibiotics have failed. Follow your veterinarian's instructions exactly.
- Administration: If provided as human granules (packets), dissolve the entire contents of a single-dose sachet in 1/2 cup (90-120 mL) of cool or room-temperature water immediately before giving it to your pet. Do not use hot water.
- Food: Can be given with or without food. If it causes stomach upset, giving it with a small treat or meal may help.
- Side Effects: The most common side effect is diarrhea. Contact your veterinarian if your pet develops severe or bloody diarrhea, vomiting, or loss of appetite.
- Toxicity Warning: This drug can be highly toxic to the kidneys in cats. If your cat was prescribed this medication under special circumstances, monitor them extremely closely for changes in thirst, urination, or lethargy, and attend all scheduled blood test appointments.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.