Monurol: How a Single Dose Treats a Bladder Infection
Monurol is a brand of fosfomycin trometamol, an antibiotic that stands out for one reason: it is taken as a single dose. One sachet of granules dissolved in water, and the course is finished.
That convenience is genuine and it comes with limits that are frequently misunderstood, including in the earlier version of this article. Getting those limits right matters, because using it for the wrong infection means an infection that goes untreated.
💧 Why one dose is enough, and what that implies. Fosfomycin is excreted almost entirely into the urine and stays there at high concentration for two to three days after a single sachet. The bladder is therefore bathed in antibiotic for the whole period needed. The corollary is that it works where urine reaches and nowhere else — which is precisely why it treats a bladder infection and does not treat a kidney infection, a prostate infection, or anything outside the urinary tract.
✅ What it is actually for
Fosfomycin trometamol is licensed for uncomplicated infection of the bladder — cystitis — in women. That is the whole of its routine indication.
It has three further uses worth knowing.
In pregnancy. It is considered acceptable in pregnancy, which matters because urinary infection in pregnancy is treated more thoroughly than outside it and the antibiotic choices are constrained. A single dose is also easier to complete than a week of tablets.
Against resistant bacteria. Fosfomycin retains activity against many organisms that have become resistant to other oral antibiotics, including some that produce the enzymes making them resistant to common penicillins and cephalosporins. In an era where resistant urinary infections increasingly leave no oral option at all, this is the reason interest in an old drug has revived.
Before urological procedures, as a single preventive dose.
🔴 It does not treat a kidney infection, and the earlier version of this article said that it did. Pyelonephritis — fever, one-sided back or flank pain, shaking chills, nausea and vomiting, feeling genuinely unwell — requires an antibiotic that penetrates kidney tissue and a course of seven days or more. A single dose of fosfomycin concentrates in urine, not in tissue, and treating a kidney infection with it means the infection continues while the person believes it has been dealt with. The same applies to infections in men, where the prostate is frequently involved. Fever with urinary symptoms means the situation has changed and needs same-day assessment.
📏 The dose, and the correction
The adult dose is one 3 gram sachet, taken once. That is the whole treatment.
Some markets also supply a 2 gram sachet, used at lower doses in specific circumstances rather than as the adult treatment dose.
The earlier version stated that elderly patients or those with more severe infections take two doses daily. That is not correct and is worth correcting plainly, since it would mean a large excess. Where a second dose is used at all — in some complicated cases or in men, under specialist direction — it is given two to three days after the first, not twice a day.
🌙 How to take it properly
The instructions look fussy and each one exists for a reason, since a single dose leaves no margin for getting it wrong.
| Instruction | Why |
|---|---|
| Dissolve the whole sachet in a glass of cold water | Not hot water, and drink it immediately once dissolved rather than letting it stand |
| Take it on an empty stomach | Food substantially delays and reduces absorption. Two to three hours after a meal, or well before one |
| Empty your bladder first | So the arriving antibiotic is not diluted by urine already sitting there |
| Take it at bedtime | The most important instruction. Overnight the bladder is not emptied for hours, so the drug sits in contact with the infected lining for the longest possible time |
| Do not take antacids or calcium supplements around it | They bind it and reduce absorption |
Taken in the morning before a working day of frequent trips to the bathroom, the same sachet delivers considerably less. The bedtime instruction is doing real work.
⏱️ What to expect afterwards
Improvement usually begins within a day and symptoms typically settle over two to three days. That is slower than people expect from a treatment described as single-dose, and the gap causes unnecessary worry.
Some burning and urgency can persist for a day or two after the bacteria have been dealt with, because the bladder lining is inflamed and takes longer to settle than the infection does.
Symptoms not improving after three days, or worsening at any point, means seeking advice rather than waiting or taking a second sachet independently. The usual explanations are a resistant organism or an infection that was never confined to the bladder.
⚖️ How it compares
Honest comparison matters here, because convenience is not the only consideration.
| Fosfomycin | Nitrofurantoin | Trimethoprim with sulfamethoxazole | |
|---|---|---|---|
| Course | Single dose | 5 days, twice daily | 3 days, twice daily |
| Completion | Guaranteed | Depends on the person | Depends on the person |
| Resistance | Low | Low after decades of use | High in many regions |
| Kidney infection | No | No | Yes, at treatment doses |
| Pregnancy | Acceptable | Yes, but avoided in the final weeks | Avoided in the first trimester and at term |
| Cure rate | Somewhat lower in direct comparison | Somewhat higher | Comparable where resistance is low |
That last row is the finding most often left out. A randomised trial comparing a single dose of fosfomycin against five days of nitrofurantoin found a higher rate of clinical resolution with the five-day course. The difference was not enormous and it was real.
So the choice is a genuine trade-off rather than a free convenience. A single dose that is definitely taken may still outperform a five-day course that is abandoned on day two, which is a common outcome once symptoms settle. Where someone will reliably complete a course, the course does slightly better.
⚠️ Side effects and cautions
Commonly: diarrhoea, nausea, headache, and a burning sensation in the chest. Most are mild and short-lived, as one would expect from a single dose. Diarrhoea is the most frequent.
Uncommonly: rash, dizziness, abdominal discomfort, vaginal thrush.
Rarely: severe allergic reaction, and prolonged or severe diarrhoea that could indicate a bowel infection following antibiotic use.
Three specific cautions.
Metoclopramide, used for nausea, lowers the concentration fosfomycin achieves and should not be taken around it. Other drugs that speed gut transit have the same effect.
Sugar content. The granules contain a substantial amount of sucrose, which anyone with diabetes should account for, and which makes the preparation unsuitable in rare inherited sugar intolerances.
Kidney function. Since the drug is cleared by the kidneys and works by concentrating in urine, significantly reduced function means it neither clears normally nor reaches useful urinary levels. It is not used where function is severely impaired.
📋 The practical position
Fosfomycin is a good option for uncomplicated cystitis, particularly where completing a course is a realistic concern, in pregnancy, and where resistance has narrowed the alternatives.
It is the wrong choice for anything above the bladder, for men, and for anyone with fever or feeling systemically unwell.
It is not stocked here, since it is supplied as sachets rather than tablets and is not part of this catalogue.
The first-line alternatives are. Treatment for uncomplicated bladder infection is available in the antibiotics category, including Macrobid (nitrofurantoin 100 mg), which is the agent fosfomycin is most directly compared against and which performed slightly better in that comparison, and Bactrim (sulfamethoxazole with trimethoprim). The article on urinary tract infections covers how the choice is made and when a bladder infection is not what you are dealing with.
❓ Frequently asked questions
How can one dose treat an infection?
Because fosfomycin is excreted almost entirely into the urine and stays there at high concentration for two to three days after a single sachet, so the bladder is bathed in antibiotic for the whole period needed. The corollary is that it works where urine reaches and nowhere else, which is why it treats a bladder infection and nothing above it.
Does it treat a kidney infection?
No, and this is the most important thing on the page. Pyelonephritis needs an antibiotic that penetrates kidney tissue and a course of seven days or more. Fosfomycin concentrates in urine rather than tissue. Fever, one-sided back or flank pain, shaking chills, nausea or feeling genuinely unwell mean the situation has changed and needs same-day assessment rather than a sachet.
When exactly should I take it?
At bedtime, on an empty stomach, after emptying your bladder. Each part matters. Food substantially reduces absorption. Emptying first stops the arriving antibiotic being diluted. And overnight the bladder is not emptied for hours, so the drug stays in contact with the infected lining for the longest possible time. Taken in the morning, the same sachet delivers considerably less.
How long until I feel better?
Improvement usually begins within a day and symptoms settle over two to three days, which is slower than people expect from something described as single-dose. Some burning and urgency can persist afterwards because the bladder lining is inflamed and takes longer to settle than the infection does. No improvement after three days, or worsening at any point, means seeking advice.
Can I take a second sachet if it has not worked?
Not on your own. Where a second dose is used at all, in some complicated cases and in men under specialist direction, it is given two to three days after the first rather than daily. If symptoms have not improved after three days the likely explanations are a resistant organism or an infection that was never confined to the bladder, and both need assessing rather than repeating the dose.
Is it better than a five-day course?
More convenient, slightly less effective. A randomised trial comparing a single dose of fosfomycin against five days of nitrofurantoin found a higher rate of clinical resolution with the five-day course. The difference was modest and real. A single dose that is definitely taken may still beat a five-day course abandoned on day two, which is a common outcome once symptoms settle.
Can it be used in pregnancy?
It is considered acceptable in pregnancy, which matters because urinary infection in pregnancy is treated more thoroughly than outside it and the antibiotic options are constrained. A single dose is also easier to complete. As with any antibiotic in pregnancy it should be prescribed for that pregnancy rather than taken from a previous supply.
Why is it used for resistant infections?
Because fosfomycin retains activity against many organisms that have become resistant to other oral antibiotics, including some producing the enzymes that defeat common penicillins and cephalosporins. As resistant urinary infections increasingly leave no oral option at all, an old drug with an unusual mechanism has become useful again.
Are there interactions to know about?
Metoclopramide, taken for nausea, lowers the concentration fosfomycin achieves and should not be taken around it, and other drugs speeding gut transit do the same. Antacids and calcium supplements bind it and reduce absorption. The granules also contain a substantial amount of sucrose, which anyone with diabetes should account for.
If you do not stock it, what should I use instead?
For uncomplicated cystitis, nitrofurantoin for five days is first line in most guidelines and is the agent fosfomycin is directly compared against — performing slightly better in that comparison. Trimethoprim with sulfamethoxazole for three days is the other first-line option where local resistance is low enough. Neither treats a kidney infection, which needs a different agent entirely.
📑 Sources and editorial
- Prescribing information for fosfomycin trometamol, including administration instructions and interactions
- IDSA and European Association of Urology guidance on uncomplicated cystitis
- Randomised trial comparing single-dose fosfomycin with five-day nitrofurantoin for uncomplicated urinary tract infection
- Reviews of fosfomycin activity against multidrug-resistant urinary pathogens
- Evidence on fosfomycin use in pregnancy and in surgical prophylaxis before urological procedures
- Pharmacokinetic studies of urinary fosfomycin concentrations after a single oral dose
- Related reading: urinary tract infections, urethritis
- Related products: antibiotics category
- RXshop Editorial Team — reviewed by Emily Chen, MD, MPH, Internal Medicine
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.