Fosfomycin: Uses, Dose, Side Effects, and Practical Tips

Fosfomycin is a single-dose, broad-spectrum antibiotic often used to treat uncomplicated urinary tract infections (UTIs). Because you usually take one 3 g sachet by mouth, it's popular for quick, simple UTI treatment and for people who want a short, convenient course.

What fosfomycin does is block a key step in bacterial cell wall construction, which kills many common UTI bacteria including E. coli. It's chemically different from penicillins and fluoroquinolones, so it can work when other drugs fail. The oral form marketed as fosfomycin trometamol reaches the bladder quickly and achieves high urinary concentrations.

When to use fosfomycin

For most adults, fosfomycin is best for uncomplicated lower UTIs — bad-burning pee, urgency, frequency, or lower abdominal pressure. Pregnant women often get it as a safe option when needed. Men, people with fever, flank pain, or signs of kidney infection usually need more testing and longer treatment than a single dose. If you have a complicated urinary tract, a catheter, kidney stones, or recurrent infections, your doctor will likely choose a different plan.

How to take it: mix the 3 g sachet in water and drink it right away. You can take it with or without food. If your doctor prescribes repeated doses (sometimes used for resistant bugs), follow their exact schedule. Don't split sachets, and finish any longer course your doctor orders.

Common side effects are mild: diarrhea, nausea, headache, and vaginitis in women. Severe allergic reactions are rare but require immediate medical help — look for hives, swelling, or trouble breathing. If diarrhea is severe or contains blood, contact your provider.

Fosfomycin has few drug interactions compared with many antibiotics. Still, tell your doctor about other medicines, especially for kidney disease. Kidney function affects some antibiotic choices and dosing.

Resistance, alternatives, and real-world tips

Resistance is growing for many antibiotics. Fosfomycin can work against some resistant strains, but local resistance patterns matter. If a urine culture shows the bug is resistant, your clinician will switch to a targeted antibiotic like nitrofurantoin, trimethoprim-sulfamethoxazole, or a fluoroquinolone when appropriate. For complicated infections, IV options may be needed.

Antibiotics can disrupt your gut. A probiotic with Lactobacillus or Saccharomyces boulardii can reduce antibiotic-related diarrhea — start during treatment and continue a week after. Drink fluids, avoid alcohol while sick, and get a urine test if symptoms return within a few weeks. When in doubt, ask your doctor rather than self-treating, especially for men or people with recurrent infections.

Expect to feel better within 24 to 72 hours after taking fosfomycin for uncomplicated UTI. If symptoms don't improve in 48 to 72 hours or if you develop fever or flank pain, contact your clinician — that could mean a deeper infection. Keep a urine sample for testing if asked. Also, keep the original sachet packaging until treatment is done and note the batch number if you have a bad reaction. Avoid unnecessary antibiotics; only use fosfomycin when prescribed. Let lab results and local resistance patterns guide any re-treatment. Save questions for your healthcare provider.