8 Fosfomycin Side Effects You Should Know About

Fosfomycin (Monurol) is an antibiotic that’s approved to treat urinary tract infections (UTIs) in women. You only need one dose of fosfomycin to treat a UTI. It’s generally well-tolerated, but there are still some side effects you should be aware of before you take it.

Fosfomycin side effects at a glance

Most fosfomycin side effects are mild, but there are also serious side effects to be aware of. Below is a list of side effects reported in clinical trials. Keep in mind that everyone responds to medication differently, so you may experience side effects that aren’t listed here. It’s best to check in with your prescriber about any side effects you're concerned about.

Mild side effects:

• Diarrhea

• Headache

• Vaginal inflammation or infection

• Nausea

• Runny nose

• Back pain

• Menstrual cramps

• Dizziness

• Stomach pain

• General weakness

• Heartburn

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Rare but serious side effects:

• Clostridioides difficile diarrhea

• Severe allergic reaction

• Low blood cell counts

• Asthma flare-up

• Yellowing of the skin (jaundice)

• Liver problems

1. Diarrhea

Diarrhea is the most common fosfomycin side effect. It happens because fosfomycin may kill off some of the good bacteria in your gut. Most of the time, diarrhea is mild and will go away on its own in a few days.

Taking a probiotic can help reduce the chance of diarrhea. If needed, an over-the-counter (OTC) medication such as loperamide (Imodium A-D) or bismuth subsalicylate (Pepto-Bismol, Kaopectate) may also help. And be sure to drink plenty of fluids, since diarrhea can dehydrate you.

Good to know: In rare cases, fosfomycin (as well as other antibiotics) can cause a severe form of diarrhea called Clostridioides difficile (C.diff) colitis. It happens when C.diff bacteria overgrow and cause an infection in your gut. This infection can happen with other antibiotics too and may be life-threatening if not treated promptly. Signs of C.diff  include severe watery or bloody stools, abdominal cramps, and fever. Seek medical care right away if you have severe diarrhea in the days or weeks following your fosfomycin dose. 

2. Headache

Headaches are a common side effect of many medications, including fosfomycin. You can help combat a mild headache by staying well hydrated or trying meditation or breathing exercises. If needed, talk to your pharmacist or prescriber about an OTC pain medication, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol).

If your headache is severe or doesn’t go away, be sure to let your healthcare team know. They may want to see you to make sure there isn’t something else causing it.

3. Vaginal irritation or infection

Vaginal inflammation or infection (vaginitis) is another possible fosfomycin side effect. The antibiotic can kill off healthy bacteria in your vagina, which can lead to irritation or infection in some cases. Symptoms may include:

• Vaginal burning or itching

• Unusual vaginal discharge

• A change in vaginal odor

• Rashes, lumps, or bumps

A probiotic may help prevent or manage vaginitis. It’s also a good idea to avoid tight clothing, douching, and fragranced soaps. A mild vaginal infection may go away on its own. But if your symptoms are bothering you, or they aren’t going away, talk to your prescriber about treatment options.

4. Nausea and heartburn

Nausea and heartburn can also occur with fosfomycin. Since it’s taken as just one dose, these symptoms should resolve in a short amount of time. You can also eat something before taking your dose to help reduce nausea.

If needed, an OTC nausea medication may help lessen your symptoms. Ginger or peppermint tea or lozenges can also help settle your stomach. If nausea is severe, or you vomit soon after taking fosfomycin, be sure to let your prescriber know. You may need another antibiotic for your UTI.

Keep in mind: UTIs can also cause nausea, especially if your kidneys are affected. Fosfomycin doesn’t treat kidney infections. So make sure to let your prescriber know if you have severe nausea along with a fever or pain in your stomach or back. They can make sure it’s not related to a worsening infection.

5. Dizziness

Dizziness isn’t common with fosfomycin, but it happens in some cases. You may feel unsteady, especially when moving from a seated to standing position. So it’s best to move slowly, and avoid driving or other activities that require coordination until you know how fosfomycin affects you. And stay hydrated, as low fluid levels can make dizziness worse. Let your prescriber know if dizziness worsens or doesn’t go away.

6. Allergic reaction

As with any antibiotic, it’s possible to have an allergic reaction to fosfomycin. This may be a mild reaction, such as an itchy skin rash. It can also be more severe, causing swelling of the mouth and throat or trouble breathing.

It’s best to report any signs of a mild allergic reaction to your healthcare team. They can let you know what to watch for and when to seek care. If you notice swelling around your mouth or throat or start to have trouble breathing, seek emergency care right away.

7. Low blood cell counts

It’s possible for fosfomycin to lower the amount of blood cells in your body (aplastic anemia). This is a very rare side effect, but it’s good to be aware of, as this can be serious.

Symptoms include bruising easily, infections that aren’t going away, and feeling more tired than usual. Again, this isn’t common, but if you notice these symptoms after taking fosfomycin, it’s best to let your healthcare team know. They can further investigate to determine the cause.

8. Liver problems

Another rare fosfomycin side effect is that it can affect your liver. In some cases, your liver enzymes may become elevated. In others, you may have an increased level of bile, which can make your skin and eyes turn yellow (jaundice). 

You may not have any noticeable symptoms if this side effect happens. And in most cases, these issues aren’t serious and will go away on their own. But let your healthcare team know if you notice a yellowing of your skin or eyes, itchy skin, or severe fatigue.    

When should you seek medical care for fosfomycin side effects?

If any fosfomycin side effects are bothering you or don’t go away on their own, it’s a good idea to mention them to your healthcare team. You should also let them know if you develop signs of a vaginal infection or a fever. Or if your UTI symptoms haven’t improved 2 or 3 days after taking fosfomycin.

Serious side effects, such as severe diarrhea, trouble breathing, or yellow skin, require medical care right away.

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Written by:

Jill Barat, PharmD

Jill Barat, PharmD has nearly 10 years of experience as a pharmacist, including serving as the pharmacist in charge at an independent sterile and non-sterile compounding pharmacy. She also worked as the head of medical content for a telemedicine company.

Edited by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

Reviewed by:

Daphne Berryhill, RPh

Daphne Berryhill, RPh, has two decades of experience as a clinical pharmacist. She spent most of her career in the Chicago area practicing in-home infusion.

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