Fosfomycin
Is It Right for You?
A complete guide to Fosfomycin — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Fosfomycin?
Fosfomycin is an orally administered antibiotic with a unique chemical structure and mechanism of action distinct from all other antibiotic classes. The oral formulation used for urinary tract infections is fosfomycin trometamol (trometamine salt), available as a 3 g sachet of granules dissolved in water. It is most commonly prescribed as a single-dose treatment for uncomplicated lower urinary tract infection (cystitis) in women, making it highly convenient and associated with excellent adherence. Brand names include Monurol (UK/international).
Fosfomycin has a broad spectrum of antibacterial activity, including excellent activity against the most common urinary pathogens: Escherichia coli, Enterococcus faecalis, Klebsiella pneumoniae, and Staphylococcus saprophyticus. Importantly, it retains activity against many multidrug-resistant organisms including extended-spectrum beta-lactamase (ESBL)-producing E. coli — a growing public health problem in UTI management. Its unique mechanism means there is no cross-resistance with other antibiotic classes, making it particularly valuable when other first-line UTIs antibiotics are resistant or contraindicated.
NICE recommends fosfomycin trometamol 3 g as a single dose as one of the first-line antibiotic options for uncomplicated lower UTI in women, alongside nitrofurantoin and pivmecillinam.
What Conditions Does Fosfomycin Treat?
Fosfomycin is prescribed for:
Fosfomycin is considered a safe and effective option for uncomplicated UTI during pregnancy. UTIs in pregnancy carry significantly increased risks of pyelonephritis, preterm labour, and foetal growth restriction if untreated, making prompt and safe antibiotic treatment essential. At the single 3 g dose, fosfomycin has not been associated with significant adverse effects on pregnancy outcomes in available studies, and it represents a useful alternative when nitrofurantoin (avoided near term) or trimethoprim (avoided in the first trimester) are not appropriate. Clinical guidance should always be followed.
Extended-spectrum beta-lactamase (ESBL)-producing E. coli are resistant to most standard oral antibiotics used for UTI, including many penicillins, cephalosporins, trimethoprim, and quinolones. Fosfomycin, which has a mechanism entirely unrelated to beta-lactam antibiotics, often retains activity against ESBL-producing organisms. It is therefore a valuable option in patients with culture-confirmed ESBL UTI who can be managed in the community with oral therapy. The sensitivity of the specific ESBL-producing organism to fosfomycin should ideally be confirmed by sensitivity testing before relying on this treatment.
In complicated UTI (involving structural or functional urinary tract abnormality, catheterisation, renal impairment, or immunosuppression) or UTI in men, longer courses or alternative regimens are generally required. Multiple-dose fosfomycin regimens (3 g every 48–72 hours for 3–7 days or longer) are sometimes used in specialist settings for complicated UTI or catheter-associated UTI. Intravenous fosfomycin (a different formulation) is used in hospital for severe or resistant infections, including those caused by MRSA.
While Fosfomycin is effective for managing uncomplicated urinary tract infections, it should be used cautiously in patients with a history of hypersensitivity or severe renal impairment.
How Does Fosfomycin Work?
Fosfomycin inhibits the bacterial enzyme MurA, which is essential for bacterial cell wall synthesis. By permanently inactivating this enzyme, fosfomycin disrupts the ability of bacteria to maintain their cell wall, leading to bactericidal activity. This unique mechanism makes fosfomycin particularly valuable when other antibiotics are ineffective due to resistance.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Always follow your clinician's instructions. Dissolve one 3 g sachet in approximately 90–120 ml of cold or room-temperature water and drink immediately. It can be taken with or without food.
Side Effects of Fosfomycin
Fosfomycin is generally well tolerated, but some side effects may occur.
- Diarrhoea: The most frequently reported side effect; usually mild and self-limiting.
- Nausea and vomiting
- Abdominal pain or discomfort
- Headache
- Vulvovaginal candidiasis (thrush): Antibiotic-related disruption of vaginal flora; manageable with OTC antifungals.
- Clostridium difficile-associated diarrhoea: Risk is substantially lower than with broad-spectrum antibiotics, reflecting fosfomycin's relatively narrow microbiome impact, but remains a theoretical consideration.
- Severe allergic reactions: Including anaphylaxis — rare but reported.
- Elevated liver enzymes: Transient; uncommon at single-dose treatment levels.
- Asthma exacerbation: Very rare; reported in patients with pre-existing asthma.
Patients should be informed about the risk of allergic reactions and the importance of not using Fosfomycin for upper urinary tract infections.
Drug Interactions
Fosfomycin has a limited drug interaction profile, but it is important to avoid taking it with metoclopramide, as it can reduce the absorption of fosfomycin.
Additive anticoagulant and antiplatelet effects significantly increase the risk of gastrointestinal and systemic bleeding. Avoid concurrent use unless clinically necessary and closely monitored.
Concurrent use substantially increases the risk of gastrointestinal ulceration and bleeding. Avoid combination of multiple NSAIDs.
Additive gastrointestinal irritation increases the risk of ulcers and bleeding. Use cautiously with gastroprotection where necessary.
NSAIDs may reduce antihypertensive effectiveness and may impair kidney function. Monitor blood pressure and renal function.
Important Warnings
Always dissolve the entire sachet in water before taking. Do not take the granules dry or mixed with other liquids. Incorrect preparation may reduce drug concentration and treatment efficacy.
Avoid taking metoclopramide (an anti-nausea or prokinetic medication) at the same time as fosfomycin, as it significantly reduces the urinary concentration of fosfomycin and may result in treatment failure.
The single 3 g dose is indicated for uncomplicated lower UTI (cystitis) only. If you develop fever, loin or back pain, rigors, or feel systemically unwell, this may indicate kidney infection (pyelonephritis), which requires a different antibiotic regimen and urgent medical assessment. Single-dose fosfomycin is not adequate treatment for upper UTI.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Fosfomycin FAQs
Yes. Single-dose fosfomycin 3 g achieves urinary fosfomycin concentrations far exceeding the MIC of susceptible urinary pathogens for 24–48 hours — long enough to eradicate the infection from the lower urinary tract in clinical trials with cure rates equivalent to standard 3–7 day antibiotic courses for uncomplicated cystitis. The convenience of a single dose significantly improves patient adherence compared with multi-day regimens.
Fosfomycin's key advantages are: a unique mechanism of action with no cross-resistance to other antibiotic classes; activity against ESBL-producing E. coli and other multidrug-resistant organisms; single-dose treatment simplicity; a low drug-interaction profile; and suitability during pregnancy. It is one of the few antibiotics that retains reliable activity against the increasing proportion of community-acquired UTIs caused by antibiotic-resistant organisms.
Fosfomycin is primarily a treatment for acute uncomplicated cystitis episodes. It is not typically used as a long-term prophylactic agent in the same way that low-dose nitrofurantoin is used for recurrent UTI prevention. Each recurrent episode can be treated with a single 3 g dose if the causative organism is susceptible. The management of recurrent UTIs (3 or more per year) should involve clinician review to assess contributing factors, bladder hygiene, and whether prophylactic strategies are appropriate.
The available data on fosfomycin use during pregnancy, including in the first trimester, have not identified a consistent signal for adverse pregnancy outcomes or teratogenicity. Unlike trimethoprim (which is a folate antagonist and avoided in the first trimester) and nitrofurantoin (avoided near term), fosfomycin does not have trimester-specific contraindications based on established mechanisms of harm. However, as with all medications in pregnancy, clinical judgement is required. Always inform your clinician that you are pregnant when requesting any antibiotic.
It is normal for some burning and urinary frequency to persist for 24–48 hours after a single fosfomycin dose while the infection clears. Symptoms typically begin to improve within 24 hours and should resolve fully within 2–3 days. If symptoms are not improving after 3 days, are getting worse, or if you develop fever or loin pain, contact your clinician — you may require a urine culture and a different or extended antibiotic course.