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Fosfomycin: Uses, Dosage & Safety Information

Fosfomycin

Fosfomycin
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Fosfomycin

Fosfomycin tromethamine
Fosfomycin is a prescription antibiotic used primarily to treat uncomplicated urinary tract infections (UTIs), such as cystitis. Its unique feature is a single-dose regimen, where one packet of fosfomycin tromethamine powder is dissolved in water and taken orally.
  • ActiveIngredient: Fosfomycin tromethamine
  • DosageForm: Granules for oral solution
  • Dosage: Single-dose sachet: 3 g
  • Indications: Uncomplicated urinary tract infection (acute cystitis) in women aged 18 and over
  • Manufacturer: Generic manufacturers
  • Storage: Store as directed on the sachet; keep out of reach of children
  • Drug Status: Prescription Only
Drug Status: Prescription Only
Description
One indication only
Fosfomycin is licensed for a single condition: uncomplicated urinary tract infection in women.
The label states explicitly that it is not indicated for pyelonephritis or perinephric abscess. A kidney infection needs a different antibiotic, not a second sachet.1

Fosfomycin tromethamine is a prescription antibacterial supplied as granules in a single-dose sachet. It treats acute cystitis in women aged 18 and over caused by susceptible Escherichia coli or Enterococcus faecalis, and one 3 g sachet mixed into water is the entire course.1 It is listed under antibiotics on iMedix.

It is never swallowed dry. The granules are mixed with water before drinking — the label makes that an instruction, not a suggestion.

If bacteriuria persists or returns after treatment, the label directs choosing a different agent rather than repeating this one.

General Instructions

Mix with water first. Pour the whole sachet into water, stir until dissolved and drink it. Dry granules are not swallowed.1

Food does not matter. It can be taken with or without a meal.

One dose is the course. There is nothing to finish and nothing to remember tomorrow.

If symptoms persist after a few days, go back rather than repeat. Persistent or returning bacteriuria calls for a different antibiotic.

Side Effects
Reported Side Effects
SystemEffectWhat the label says
GastrointestinalDiarrhoeaAmong the reactions reported with single-dose treatment
GastrointestinalNausea, dyspepsia, abdominal painReported
Nervous systemHeadache, dizzinessReported
GenitourinaryVaginitisReported
SkinRashReported
GastrointestinalClostridioides difficile-associated diarrhoeaReported with nearly all antibacterials, ranging to fatal colitis

Source: 1

Emergency – stop and seek care
  • Watery or bloody diarrhoea, during the days after the dose or weeks later
  • Swelling of the face, lips or tongue, or difficulty breathing
  • Fever with flank pain or vomiting — signs the infection has reached the kidney
Uses

One licensed indication, defined narrowly.1

What it is not for. The label names two exclusions outright: pyelonephritis and perinephric abscess. Both are infections of the kidney rather than the bladder, and both need different treatment.

The single-dose format is the drug’s reason for existing: one sachet, one course, no adherence problem. That advantage disappears if it is used for an infection it was not designed to reach.

Safety Advice

Interactions Alcohol:

Consult your doctor
The label sets no alcohol restriction. With a single dose the practical window is short, but alcohol irritates the bladder and can worsen the symptoms being treated.

Interactions Other Medications:

Use with caution
Metoclopramide lowers both the blood concentration and the urinary excretion of fosfomycin, and other drugs that speed gastrointestinal motility may do the same. Since the drug works by reaching the urine, that matters more than it sounds.

Special Groups Pregnancy:

Consult your doctor
The prescribing information does not establish a recommendation for pregnancy. Urinary infection in pregnancy is itself a reason for prompt treatment, so the choice of agent belongs to the prescriber.

Special Groups Breastfeeding:

Consult your doctor
The label does not establish a breastfeeding recommendation. With a single dose, timing is usually the only question.

Special Groups Elderly:

Use with caution
No separate geriatric regimen is set. In older women, urinary symptoms more often reflect something other than simple cystitis, which is where the label exclusions matter.

Special Groups Children:

Consult your doctor
The indication covers women aged 18 and over. The label provides no paediatric regimen, so use below that age is outside the approved labelling.

Effects on Activities Driving:

Safe if prescribed
Dizziness and headache are among the reported reactions, but a single dose for cystitis does not ordinarily affect driving. Judge by how you feel after taking it.

Effects on Activities Operating Machinery:

Safe if prescribed
The same applies. The drug is not sedating, and the course lasts one dose.
Concerns

Fosfomycin is prescription-only and supplied as single-dose granules. The convenience of one sachet is also its main clinical argument: no course to complete, no doses to miss.

The concern worth naming is scope. Because it is easy to take, it can be reached for when symptoms suggest more than cystitis. The label draws that line explicitly, excluding kidney infection.1

Warnings

Not for kidney infection

The label excludes pyelonephritis and perinephric abscess by name. Fever, flank pain or vomiting alongside urinary symptoms point away from simple cystitis, and a single sachet is the wrong treatment for them.

Persistent or recurrent infection

If bacteriuria persists or returns after treatment, another agent is selected. Repeating fosfomycin is not the label’s answer.

Antibacterial-associated colitis

Nearly all antibacterials have been associated with C. difficile colitis, ranging in severity to fatal. Diarrhoea after treatment is worth reporting rather than waiting out.

Preparation matters

The granules are dissolved in water before drinking. Taking them dry is specifically warned against.

Source: 1

Dosage

Fosfomycin Dosage Information

The following information represents the standard adult dosage for its primary indication. Always follow your doctor’s specific prescription.

Standard Fosfomycin Dosage for Uncomplicated Cystitis
ConditionStandard DosageForm & Administration
Uncomplicated Urinary Tract Infection (Acute Cystitis) in Women3 grams (one single-dose packet)Oral granules for solution. Dissolve the entire contents of one packet in 90-120 mL of cold water and drink immediately. This is typically a one-time dose.

Important Administration Notes

  • Single Dose: The standard treatment for uncomplicated cystitis is one packet. Do not repeat the dose unless instructed by your doctor.
  • Renal Impairment: No specific dosage adjustment is recommended for mild to moderate impairment. Use in severe renal impairment is not recommended.
  • Missed Dose: This is a single-dose therapy, so the concept of a “missed dose” does not apply. If you forget to take it, take it as soon as you remember the same day. If it is the next day, contact your doctor for advice.
▶Clinical detailstechnical detail

Fosfomycin is a phosphonic acid derivative that blocks MurA, the enzyme catalysing the first committed step of peptidoglycan synthesis. Because the target sits earlier in the pathway than the targets of beta-lactams, cross-resistance with penicillins and cephalosporins is uncommon.

The single-dose regimen depends on renal handling: the drug is excreted unchanged and concentrates in urine, where it remains above inhibitory concentrations for a prolonged period after one 3 g dose. This is also why anything that hastens gastrointestinal transit — metoclopramide, for instance — matters more than it would for a drug given repeatedly: less absorption means less drug reaching the bladder.1

Interactions
Significant Interactions
Interacting substanceEffect and riskSeverityDocumentation
MetoclopramideLowers serum concentration and urinary excretion of fosfomycin; other drugs that speed gastrointestinal motility may do the sameMajorGood
Hypersensitivity to fosfomycinContraindication to useContraindicatedExcellent

Source: 1

The interaction list is short because the exposure is short: one dose, cleared by the kidney. What matters is anything that moves it through the gut faster than it can be absorbed.

FAQs
Most people notice improvement within two to three days of the single sachet. If symptoms persist beyond that, or if fever or flank pain appear, the answer is to go back rather than take another dose: the label directs choosing a different agent when bacteriuria persists or returns.
For uncomplicated cystitis in women aged 18 and over, yes. The approved dosage is a single 3 g sachet, and that is the entire course. This is the drug’s main clinical argument: nothing to complete, no doses to miss.
Pour the whole sachet into water, stir until dissolved and drink it. The label instructs that it is never taken in dry form. Food does not matter — it can be taken with or without a meal.
The elimination half-life is roughly 5 to 6 hours, but what matters is urine rather than blood: the drug is excreted unchanged and concentrates in the bladder, staying above inhibitory levels long enough for one dose to do the work of a multi-day course.
No. The label excludes pyelonephritis and perinephric abscess by name. Those are infections of the kidney rather than the bladder, and they need a different antibiotic and usually a longer course. Fever, flank pain or vomiting alongside urinary symptoms point that way.
Diarrhoea is the most commonly reported, followed by nausea, headache, dizziness and vaginitis. Watery or bloody diarrhoea during the following days or weeks is different: nearly all antibacterials have been associated with Clostridioides difficile colitis, and that warrants medical attention.
Evidence and Reported Data

Evidence scope

What we know
  • The indication is defined precisely: acute cystitis in women aged 18 and over, caused by susceptible E. coli or E. faecalis.
  • A single 3 g sachet is the complete course, taken with or without food.
  • Two conditions are excluded by name: pyelonephritis and perinephric abscess.
What remains uncertain
  • The label sets no regimen for men, for children, or for women under 18.
  • How often single-dose treatment fails against less susceptible organisms is not quantified in the labelling.
  • Interaction data are limited to effects on gastrointestinal motility.

Source: 1

What has been reported to the FDA

As of the FAERS data cut of 30 July 2026, the FDA Adverse Event Reporting System held 3,787 reports naming fosfomycin, of which 3,565 were classed as serious — 94% of the total.3

Most Reported Terms
Coded termReports
Diarrhoea655
Off label use589
Nausea561
Pyrexia558
Dyspnoea543
Malaise523
How to read this table

FAERS records what was reported, not what the medicine caused, and has no denominator – no count can become a rate.

Figures above are the counts held at the 30 July 2026 data cut, retrieved on 2 September 2026; they do not update between cuts.

Population note: the 94% serious proportion is strikingly high for a drug used in simple cystitis, and the second most frequent term explains part of it — “off label use” with 589 reports. Outside the United States fosfomycin is also given intravenously for severe infection in hospital, and those patients are sicker by definition. Reports name the product, not the culprit, and not necessarily the sachet described on this page.

Who was this medicine studied in?

The licensed evidence comes from studies in women aged 18 and over with uncomplicated cystitis.1 No Drug Trials Snapshot exists: the approval predates that programme.2

Who this is and is not appropriate for

It is not appropriate for anyone hypersensitive to fosfomycin, and not for pyelonephritis or perinephric abscess. The label provides no regimen for men or for anyone under 18.1

It suits an uncomplicated bladder infection in a woman where a single dose is the whole plan — and where fever, flank pain or vomiting are absent, because those point somewhere else.

Two practical points sit between the evidence and the prescription. First, susceptibility: the indication names E. coli and E. faecalis specifically, and cystitis caused by something else is outside what the studies covered. Second, the exclusions are not cautions but boundaries — pyelonephritis and perinephric abscess are named as conditions this drug does not treat, which is why fever or flank pain changes the answer rather than the dose.

Where to go next

References

References — verified Sep 2, 2026

  1. Fosfomycin tromethamine granules for oral solution — prescribing information. DailyMed SPL set ID 3fa61bd4-fc55-0987-e063-6394a90abce2. dailymed.nlm.nih.gov. Accessed September 2, 2026.
  2. US Food and Drug Administration. Drugs@FDA records for fosfomycin tromethamine. Accessed September 2, 2026.
  3. US Food and Drug Administration. FDA Adverse Event Reporting System, queried through openFDA for medicinalproduct FOSFOMYCIN. Data cut 2026-07-30 · Accessed 2026-09-02.

Three sources are listed because three are cited, covering FDA-approved labelling, FDA regulatory records and FDA post-marketing data. Links under “Where to go next” are routing destinations, not citations.

Disclaimer

This page is medical information, not medical advice. Fosfomycin is a prescription antibacterial licensed for one condition, and its labelling excludes kidney infection by name. Nothing here replaces assessment by the clinician treating you.