Fosfomycin Side Effects: What to Expect and When to Call Your Doctor

Fosfomycin (Monurol) is generally considered one of the best-tolerated UTI antibiotics — a key reason it is recommended as a first-line option by IDSA guidelines. Most people who take the single 3-gram dose experience only mild side effects or none at all. However, knowing what to expect helps you identify the rare cases that need medical attention.

Common Side Effects (Usually Mild and Self-Resolving)

The following side effects were reported in clinical trials of fosfomycin and are generally mild and temporary:

Diarrhea: The most commonly reported side effect. Usually mild and resolves on its own. Taking fosfomycin with food can help reduce this.

Nausea: Mild stomach upset may occur shortly after taking the dose. Typically passes within a few hours.

Headache: Reported in a small percentage of patients. Often resolves without treatment.

Vaginitis: Vaginal irritation or discharge was reported in some clinical trial participants.

Back pain: Reported in a small number of users. Usually mild.

Rhinitis (runny nose): Reported in a subset of patients.

Abdominal pain / indigestion: Mild stomach discomfort or dyspepsia, especially if taken on an empty stomach.

These side effects are generally minor and do not require stopping treatment or contacting your doctor unless they are severe or persistent.

When Should You Call Your Doctor After Taking Fosfomycin?

Your UTI symptoms don't improve after 2-3 days. You should start feeling better within 2-3 days of taking your fosfomycin dose. If your symptoms — burning, frequency, urgency — have not improved by day 3, call your prescriber. The bacteria causing your infection may be resistant to fosfomycin, or you may need a different treatment.

You develop severe or bloody diarrhea. Mild diarrhea is common. But severe, watery, or bloody diarrhea — especially with fever, stomach cramps, or fatigue — can be a sign of Clostridioides difficile (C. diff) infection, which can occur even weeks after finishing antibiotics. Seek medical care immediately.

Signs of an allergic reaction occur. Seek emergency care immediately if you experience hives, swelling of the face/lips/tongue/throat, difficulty breathing, or rapid heartbeat. These signs of anaphylaxis are rare but serious.

Symptoms get worse instead of better. If UTI symptoms worsen after taking fosfomycin, or you develop fever, chills, back/flank pain, or vomiting, this may suggest the infection has spread to your kidneys (pyelonephritis). Oral fosfomycin is not effective for kidney infections — seek prompt medical evaluation.

Serious but Rare Side Effects

These serious adverse events are rare but have been reported in post-marketing surveillance:

Clostridioides difficile-associated diarrhea (CDAD) or colitis: Can occur up to 2 months or more after antibiotics. Notify your doctor of any prolonged or severe diarrhea.

Toxic megacolon: Extremely rare complication associated with severe C. diff. Requires emergency medical care.

Anaphylaxis / anaphylactic shock: Severe allergic reaction. Rare but call 911 immediately if suspected.

Aplastic anemia: Extremely rare. Symptoms include unusual fatigue, easy bruising, or frequent infections.

Why Fosfomycin Has Fewer Side Effects Than Many Antibiotics

Fosfomycin concentrates specifically in the urinary tract. Because it reaches very high concentrations in the urine — the target site — with relatively low serum levels, it has limited impact on the gut bacteria compared to broader-spectrum antibiotics. This is why C. diff risk with fosfomycin is considered lower than with fluoroquinolones or broad-spectrum penicillins, and why gastrointestinal side effects are typically mild.

For information on drug interactions to be aware of, see: Fosfomycin Drug Interactions: What to Avoid and What to Tell Your Doctor