Fosfomycin: what it treats, how to take it, side effects
Last updated September 3, 2026.
Fosfomycin is an antibiotic that kills the bacteria behind urinary tract infections. You may know it as Monurol. The one-dose treatment: a single packet, mixed with water, once.
What does it treat?
Uncomplicated urinary tract infection - acute cystitis, the bladder infection - in women. Not for kidney infections or deeper urinary infections.
How do you take it?
Once, as a single dose, with or without food - but the mixing is mandatory: pour the entire packet into a glass, add 3 to 4 ounces (90 to 120 ml) of cold water, stir to dissolve, and drink immediately. Never hot water, never the dry granules straight. Symptoms usually improve within 2 to 3 days - if they do not, or they return, call your doctor: that pattern means the bacteria needs a different answer. Missed dose is not a concept here - it is one dose; if you are unsure you absorbed it (vomiting soon after), call your doctor.
Side effects to know
- Common: nausea, diarrhea, headache, and vaginal itching.
- Serious - call your doctor immediately: rash, itching, hives, swelling of face, tongue, or lips, and trouble breathing - the allergic set; and watery diarrhea with abdominal pain, cramping, fever, nausea, or loss of appetite - the C. diff pattern, possible during or after antibiotic treatment.
Who should not take it?
It is labeled for uncomplicated bladder infections in women - kidney infection signs (fever, back pain, vomiting) need different treatment, not this packet. Men and children are outside the labeled use. If you are pregnant or breastfeeding, the prescriber weighs it - say it plainly. If you become pregnant, call your doctor.
When is it an emergency?
Face, tongue, or lip swelling or trouble breathing is a 911 call. High fever with back pain after taking it means the infection climbed - same-day care. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
How can one dose treat an infection?
The pharmacology: the drug concentrates in urine and stays at killing levels there for a day and more - one 3-gram dose blankets the bladder. That is also why the indication is narrow: uncomplicated bladder infection in women. Kidney infections and anything deeper need multi-day drugs that reach tissue levels. One packet, one bladder, one treatment.
Why the mixing ritual - cold water, drink now?
The granules are not meant to be swallowed dry: dissolve the whole packet in 3 to 4 ounces of cold water - never hot - stir, and drink as soon as it is prepared. The water delivers the drug evenly and protects your throat and stomach. If you vomit soon after taking it, call your doctor - the dose may not have counted.
When should I feel better - and what if I do not?
Most improve within 2 to 3 days. Not improving by then, or symptoms returning after they cleared, is the call-your-doctor pattern: the bacteria may be resistant, the diagnosis may be off, or the infection may be more than a simple cystitis. Do not take a second dose on your own - the label is one dose; the next step is a culture, not a repeat.
What symptoms mean it is not just a bladder infection?
Fever, back or flank pain, nausea and vomiting - the kidney-infection set. Fosfomycin granules are not indicated for kidney infections; those signs before or after the packet mean a different, deeper treatment, promptly. Blood in the urine also deserves a mention at evaluation - common with cystitis, but it belongs on the record.
Why is it not for men or children?
The trials and the label are for uncomplicated cystitis in women - male urinary infections involve the prostate more often and need longer, deeper courses, and pediatric use is not established. The single-dose convenience is specific to its indication; outside it, other antibiotics do the work.
What happens with the diarrhea afterward?
Any antibiotic can disturb the gut: mild loose stools are common and brief. The version that matters: watery diarrhea with abdominal cramping, fever, nausea, or loss of appetite - possible during treatment or weeks after - is the C. diff pattern, and it earns an immediate call. Do not self-treat that diarrhea with anti-diarrhea medicine; call first.