Fosfomycin
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Uncomplicated urinary tract infection (acute cystitis) due to E. coli or E. faecalis | Women ≥18 years | Monotherapy (single dose) | FDA Approved |
Fosfomycin is a unique phosphonic acid antibiotic that achieves high urinary concentrations following a single 3-gram oral dose, making it one of the most convenient treatment options for uncomplicated cystitis. The IDSA 2010 guidelines list fosfomycin as one of three first-line agents for uncomplicated cystitis (alongside nitrofurantoin and TMP-SMX), with the caveat that its microbiologic eradication rates are somewhat lower than other first-line agents. Its unique mechanism of action (MurA inhibition) means there is generally no cross-resistance with other antibiotic classes, making it valuable for ESBL-producing and multidrug-resistant uropathogens.
Fosfomycin is not indicated for pyelonephritis or perinephric abscess, as oral dosing does not achieve sufficient systemic or renal parenchymal concentrations.
Complicated or MDR UTI (ESBL-producing E. coli, VRE) — 3 g PO every 48–72 hours for 3 doses. Used when other oral options are limited by resistance. Evidence quality: Low-Moderate (retrospective data).
Chronic bacterial prostatitis — 3 g PO every 48–72 hours for multiple weeks. Fosfomycin distributes to prostate and seminal vesicles, but evidence is limited. Evidence quality: Low.
Transrectal prostate biopsy prophylaxis — 3 g PO single dose before procedure. Non-inferior or superior to fluoroquinolone prophylaxis in recent studies. Evidence quality: Moderate.
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Acute uncomplicated cystitis (FDA-approved) | 3 g PO single dose | — | 3 g (one dose) | Dissolve sachet in 3–4 oz water; take immediately; can be taken with or without food FDA PI: do not use more than one dose per episode; repeated daily doses increase AEs without improving efficacy |
| MDR/ESBL UTI — oral salvage therapy | 3 g PO once | 3 g PO every 48–72 h | 3 g q48h × 3 doses | Off-label; limited evidence for multi-dose regimens; optimal duration undefined Expert opinion / retrospective data; not FDA-approved |
| Prostate biopsy prophylaxis | 3 g PO single dose | — | 3 g (one dose) | Administered 1–3 hours before transrectal biopsy Off-label; non-inferior to fluoroquinolone prophylaxis in RCTs |
Special Populations
| Population | Adjustment | Maximum Dose | Dialysis | Notes |
|---|---|---|---|---|
| Renal impairment (CrCl 7–54 mL/min) | No specific FDA adjustment | 3 g single dose | — | t½ increases from 11 to 50 h; urinary recovery decreases from 32% to 11%; clinical efficacy may be reduced in severe impairment |
| ESRD / Hemodialysis | Use with caution | 3 g single dose | t½ = 40 h on HD; drug is dialyzable | Limited data; efficacy uncertain due to markedly reduced urinary excretion |
| Elderly (≥65 years) | No dose adjustment | 3 g single dose | — | No clinically significant differences in urinary excretion observed |
| Pediatric (≤12 years) | Not established | — | — | Safety and effectiveness not established in children ≤12 years |
Fosfomycin is one of very few antibiotics where a single oral dose constitutes the full treatment course. The FDA PI explicitly warns against using more than one dose per episode, as repeated daily doses did not improve clinical success or microbiologic eradication but did increase adverse events. Urinary fosfomycin concentrations remain above the MIC breakpoint (≥64 mcg/mL for susceptible organisms) for approximately 24–30 hours after a single dose, and concentrations of ~10 mcg/mL persist at 72–84 hours post-dose. This sustained urinary exposure is key to its efficacy despite the short plasma half-life.
Pharmacology
Mechanism of Action
Fosfomycin exerts its bactericidal effect by irreversibly inhibiting MurA (UDP-N-acetylglucosamine enolpyruvyl transferase), the enzyme that catalyses the first committed step of bacterial peptidoglycan cell wall synthesis — the condensation of UDP-N-acetylglucosamine with phosphoenolpyruvate. This target is unique to fosfomycin and has no overlap with the mechanisms of beta-lactams, aminoglycosides, fluoroquinolones, or other antibiotic classes, which accounts for the absence of cross-resistance. Fosfomycin also reduces bacterial adherence to uroepithelial cells, an ancillary property that may contribute to clinical efficacy in urinary tract infections. The irreversible nature of its MurA inhibition, combined with the high and sustained urinary drug concentrations achieved after a standard 3-gram dose, results in bactericidal activity against susceptible uropathogens.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly absorbed as fosfomycin tromethamine, converted to free acid fosfomycin; bioavailability 37% (fasting), 30% (fed); Cmax 26.1 mcg/mL at ~2 h (fasting), 17.6 mcg/mL at ~4 h (fed); food reduces rate but not cumulative urinary excretion | Can be taken with or without food for UTI treatment; cumulative urinary exposure equivalent regardless of food; food delays Tmax but does not clinically reduce efficacy |
| Distribution | Vd 136.1 L; 0% protein binding; distributes to kidneys, bladder wall, prostate, seminal vesicles; bladder tissue concentration ~18 mcg/g at 3 h post-dose; crosses placenta | Zero protein binding allows entire circulating drug to be pharmacologically active; high bladder/urinary concentrations support UTI efficacy; insufficient systemic levels for pyelonephritis or bloodstream infections |
| Metabolism | Not metabolized; excreted unchanged in urine and feces | No hepatic metabolism; no CYP450 involvement; no dose adjustment needed in hepatic impairment; minimal drug interaction potential |
| Elimination | ~38% excreted in urine, ~18% in feces; mean urinary concentration 706 mcg/mL at 2–4 h, ≥100 mcg/mL for 26 h, ~10 mcg/mL at 72–84 h; total body clearance 16.9 L/h; renal clearance 6.3 L/h; t½ 5.7 h | Sustained urinary concentrations well above the susceptible MIC breakpoint (≤64 mcg/mL) for 24–30 h; renal impairment significantly reduces urinary recovery (32% → 11% as CrCl drops from 54 to 7 mL/min); drug is dialyzable |
Side Effects
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Diarrhea | 9.0% | Most common drug-related AE; generally mild and self-limiting after single dose; higher than comparators (nitrofurantoin 6.4%, TMP-SMX 2.3%, ciprofloxacin 3.1%) |
| Vaginitis | 5.5% | Comparable to other UTI antibiotics; may reflect flora disruption |
| Nausea | 4.1% | Lower than nitrofurantoin (7.2%) and TMP-SMX (8.6%) in head-to-head trials |
| Headache | 3.9% | Usually mild; resolves without intervention |
| Dizziness | 1.3% | Comparable to other agents |
| Asthenia | 1.1% | Self-limiting |
| Dyspepsia | 1.1% | Comparable to other agents |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Anaphylaxis | Very rare (postmarketing) | Minutes to hours | Emergency management; epinephrine; do not re-challenge |
| Angioedema | Very rare (postmarketing) | Hours | Discontinue; supportive care; airway management if needed |
| Aplastic anemia | Very rare (postmarketing) | Variable | CBC monitoring; hematology referral if confirmed |
| Cholestatic jaundice / hepatic necrosis | Very rare (postmarketing) | Variable | Monitor LFTs if symptomatic; discontinue if hepatotoxicity suspected |
| Toxic megacolon | Very rare (postmarketing) | Variable | Evaluate for C. difficile; surgical consultation if needed |
| Asthma exacerbation | Very rare (postmarketing) | Variable | Appropriate bronchodilator therapy |
| Hearing loss | Very rare (postmarketing; causality not established) | Variable | Audiology referral if symptoms develop |
Fosfomycin’s microbiologic eradication rate of ~82% for E. coli at 5–11 days is lower than ciprofloxacin or TMP-SMX but equivalent to nitrofurantoin. Clinical success rates are comparable across agents. Fosfomycin’s unique value lies in its single-dose convenience (maximizing adherence), its distinct mechanism (no cross-resistance), its activity against ESBL-producing organisms, and its minimal collateral damage to the microbiome compared to fluoroquinolones. The IDSA 2010 guideline endorses it as a first-line option but notes the lower efficacy data compared to some alternatives.
Drug Interactions
Fosfomycin has a remarkably limited drug interaction profile, consistent with its lack of hepatic metabolism, zero protein binding, and single-dose administration. It does not inhibit or induce CYP450 enzymes and has no known pharmacodynamic interactions with commonly prescribed medications. The only clinically relevant interaction identified in the FDA label involves drugs that increase gastrointestinal motility.
Fosfomycin’s lack of hepatic metabolism, zero protein binding, and absence of CYP450 involvement make it one of the lowest-interaction-risk antibiotics available. This is particularly valuable in elderly patients on polypharmacy or patients on warfarin, immunosuppressants, or other high-interaction-risk medications where selecting a UTI antibiotic with minimal interaction potential is clinically important.
Monitoring
- Symptom Resolution2–3 days post-dose
RoutinePatients should be informed that symptoms should improve within 2–3 days. If no improvement, re-evaluate and consider alternative therapy with broader tissue distribution. Persistence or reappearance of bacteriuria is more common with fosfomycin than with multi-day antibiotic courses. - Urine CultureBefore and after therapy
RoutineConfirm susceptibility pre-treatment. Post-treatment culture recommended if symptoms persist. If bacteriuria reappears, select an alternative agent per FDA PI guidance. - Renal FunctionBefore initiating in patients with known impairment
Trigger-basedRenal impairment significantly reduces urinary fosfomycin excretion (32% → 11% as CrCl drops). Efficacy may be compromised in patients with moderate-severe renal impairment due to insufficient urinary drug levels. - GI SymptomsDuring and after treatment
RoutineDiarrhea is the most common adverse effect (9%). If watery or bloody diarrhea develops (especially >2 months post-dose), evaluate for C. difficile. Toxic megacolon has been reported in postmarketing surveillance.
Contraindications & Cautions
Absolute Contraindications
- Known hypersensitivity to fosfomycin tromethamine or any component of the formulation
Relative Contraindications (Specialist Input Recommended)
- Severe renal impairment (CrCl <10 mL/min) — urinary drug recovery drops to ~11%, likely insufficient for therapeutic urinary concentrations; efficacy uncertain
- Suspected pyelonephritis or upper tract infection — fosfomycin lacks adequate tissue/systemic levels for upper urinary tract disease; use an agent with broader tissue distribution
- Complicated UTI with systemic signs — fever, flank pain, or hemodynamic instability require agents with systemic activity
Use with Caution
- Pregnancy — Category B; no adequate controlled studies in pregnant women; use only if clearly needed. Fosfomycin crosses the placenta
- Lactation — unknown whether excreted in human milk; caution advised
- Moderate renal impairment — t½ significantly prolonged; urinary recovery reduced but single dose may still achieve adequate levels for uncomplicated cystitis
- Repeated daily dosing — FDA PI explicitly states that repeated daily doses increase adverse events without improving outcomes; do not use multi-day dosing for a single cystitis episode
The FDA prescribing information for Monurol explicitly warns against using more than one single dose to treat a single episode of acute cystitis. Repeated daily doses of fosfomycin did not improve clinical success or microbiologic eradication rates compared to single-dose therapy but did increase the incidence of adverse events. Multi-dose regimens (e.g., 3 g every 48–72 h) are used off-label for complicated or MDR infections, but this is not an FDA-approved use.
Patient Counselling
Purpose of Therapy
Fosfomycin is a single-dose antibiotic specifically designed to treat simple bladder infections. It works by blocking bacterial cell wall production through a mechanism that is completely different from other antibiotics. One dose is the entire treatment course for an uncomplicated urinary tract infection.
How to Take
Pour the entire contents of the sachet into 3–4 ounces (half a cup) of cold or room temperature water — do not use hot water. Stir to dissolve and drink the entire mixture immediately. Do not take the powder in its dry form. The medication can be taken with or without food.
Sources
- MONUROL (fosfomycin tromethamine) Granules for Oral Solution. Full Prescribing Information. Zambon Switzerland Ltd / Allergan USA, Inc. Rev. May 2018. FDA LabelPrimary regulatory reference for fosfomycin oral: approved indication, single-dose regimen, complete PK data, clinical trial adverse event rates (N=1,233), comparative eradication data, and contraindications.
- Falagas ME, Vouloumanou EK, Samonis G, et al. Fosfomycin. Clin Microbiol Rev. 2016;29(2):321–347. DOIComprehensive review of fosfomycin covering mechanism, spectrum, PK/PD, clinical efficacy, and emerging role against MDR uropathogens including ESBL-producing Enterobacterales.
- Patel SS, Balfour JA, Bryson HM. Fosfomycin tromethamine: a review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy as a single-dose oral treatment for acute uncomplicated lower urinary tract infections. Drugs. 1997;53(4):637–656. DOIClassic drug review establishing the PK foundation and clinical efficacy data for single-dose fosfomycin in acute cystitis.
- Vardakas KZ, Legakis NJ, Triarides N, et al. Susceptibility of contemporary isolates to fosfomycin: a systematic review of the literature. Int J Antimicrob Agents. 2016;47(4):269–285. DOISystematic review demonstrating sustained high susceptibility rates of E. coli to fosfomycin globally, including ESBL-producing strains.
- Gupta K, Hooton TM, Naber KG, et al. International clinical practice guidelines for the treatment of acute uncomplicated cystitis and pyelonephritis in women: a 2010 update by the IDSA and ESMID. Clin Infect Dis. 2011;52(5):e103–e120. DOIIDSA guideline listing fosfomycin as one of three first-line agents for uncomplicated cystitis; notes lower bacterial efficacy compared to some alternatives but good clinical efficacy.
- Anger J, Lee U, Ackerman AL, et al. Recurrent uncomplicated urinary tract infections in women: AUA/CUA/SUFU guideline. J Urol. 2019;202(2):282–289. DOIAUA guideline addressing recurrent UTI management in women; discusses fosfomycin as a treatment option for acute episodes with its single-dose advantage.
- Tamma PD, Heil EL, Justo JA, et al. IDSA 2024 Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections. Clin Infect Dis. 2024;ciae403. DOIIDSA AMR guidance acknowledging fosfomycin as an option for uncomplicated ESBL-E cystitis but noting it is not suggested for pyelonephritis or complicated UTI.
- Kahan FM, Kahan JS, Cassidy PJ, et al. The mechanism of action of fosfomycin (phosphonomycin). Ann N Y Acad Sci. 1974;235:364–386. DOIFoundational paper elucidating fosfomycin’s mechanism of MurA (enolpyruvyl transferase) inhibition and its role in disrupting the first step of bacterial cell wall synthesis.
- Silver LL. Fosfomycin: mechanism and resistance. Cold Spring Harb Perspect Med. 2017;7(2):a025262. DOIDetailed review of fosfomycin’s molecular mechanism and the biological basis for its generally low but emerging resistance patterns.
- Bergan T, Thorsteinsson SB, Albini E. Pharmacokinetic profile of fosfomycin trometamol. Chemotherapy. 1993;39(5):297–301. DOIKey PK study establishing bioavailability, urinary excretion kinetics, and the pharmacologic basis for single-dose efficacy of oral fosfomycin tromethamine.
- Qiao LD, Zheng B, Chen S, et al. Evaluation of three-dose fosfomycin tromethamine in the treatment of patients with urinary tract infections: an uncontrolled, open-label, multicentre study. BMJ Open. 2013;3(12):e004157. DOIStudy evaluating the 3-dose (3g every other day) fosfomycin regimen for complicated UTI, providing PK and efficacy data for multi-dose off-label use.