Does metronidazole treat UTI?

It’s not a first-line treatment for most urinary tract infections

By Heidi Borst | Jul. 8, 2024 Medically reviewed by Jessica Swirble, Pharm.D.

Key takeaways

Metronidazole is commonly used to treat bacterial infections, parasitic infections, and sexually transmitted infections. It is not a first-line treatment for most UTIs but is effective for those caused by G. vaginalis.
The most common side effects of oral metronidazole are gastrointestinal, including nausea, vomiting, diarrhea, loss of appetite, and stomach pain.
First-line treatments for UTIs include trimethoprim-sulfamethoxazole, nitrofurantoin, fosfomycin, and cephalosporins. These antibiotics are effective against the most common causes of UTIs, including E. coli and Staphylococcus saprophyticus.

Metronidazole, the generic version of Flagyl, is a nitroimidazole antibiotic that is FDA-approved to treat a wide variety of parasitic, bacterial, and sexually transmitted infections—such as amebiasis, peritonitis, trichomoniasis, and pelvic-inflammatory disease. Though it may be effective in limited cases, metronidazole is not the first-line treatment for urinary tract infections (UTIs). Here’s what you need to know about metronidazole for UTIs, plus more effective treatment options.

Can metronidazole treat UTI?

Metronidazole can treat UTIs on rare occasions, but it’s usually not the best choice, says Greg Marchand, MD, a board-certified OB-GYN at the Marchand Institute in Mesa, Arizona. Metronidazole works against infections caused by anaerobic bacteria, such as appendicitis, liver abscesses, sinusitis, and tetanus. However, research indicates that anaerobic infections are rarely the cause of UTIs.

In fact, 85%-95% of UTIs are caused by aerobic bacteria, like E. coli (80%) or Staphylococcus saprophyticus (5% to 15%). Metronidazole is ineffective against these bacteria. In the rare case that a UTI is found to be caused by anaerobic bacteria, metronidazole might be prescribed, says Sazan Sylejmani, Pharm.D., owner and pharmacy manager at Westmont Pharmacy in Illinois.

One example of when this medication could be used to treat UTI is in women with bacterial vaginosis (BV) caused by the bacteria Gardnerella vaginalis. BV increases the risk of developing a UTI. Metronidazole is often prescribed to treat BV and is 92% to 96% effective against UTIs caused by G. vaginalis.

How long does it take for metronidazole to work?

The time it takes for an antibiotic to work depends on the severity of the infection, the type of bacteria, and the patient’s overall health. “Generally, UTI symptoms improve significantly within 48 to 72 hours with appropriate antibiotic treatment,” Dr. Sylejmani says.

Dr. Marchand adds that if the bacteria are anaerobic and susceptible to metronidazole, a typical antibiotic course is 500 mg of metronidazole tablets taken twice daily for seven days. To eliminate the infection, you must complete the full course of treatment.

“If metronidazole is working, you should see relief from your symptoms, including dysuria (pain, burning, or stinging during urination), frequent urination, and increased urgency to urinate within one week,” says Dr. Marchand. In some cases, your prescribing healthcare provider may want to send a urine culture to be sure the bacteria are gone, he adds.

Potential limitations of metronidazole for UTI

“The efficacy of metronidazole for a UTI would be limited to cases where anaerobic bacteria are involved, which is rare in uncomplicated UTI scenarios,” says Dr. Sylejmani. Moreover, metronidazole is associated with various side effects and contraindications, which might limit its use.

Common metronidazole side effects include:

Nausea
Vomiting
Diarrhea
Loss of appetite
Stomach pain or cramps
Metallic taste in the mouth
Headache
Dizziness

Serious side effects of metronidazole include:

Seizures
Meningitis
Encephalopathy
Peripheral neuropathy (numbness, tingling, or pain in the hands or feet)
Severe allergic reactions (rash, itching, swelling, severe dizziness, trouble breathing)
Signs of a new infection (persistent sore throat, fever)
Severe abdominal pain
Bloody or tarry stools
Vision problems (blurred vision, double vision, blind spots)
Mental and mood changes (confusion, irritability)
Unusual fatigue or weakness

Patients experiencing serious side effects should seek medical advice from a healthcare professional promptly.

Certain drug interactions may also cause adverse effects. Do not take metronidazole with:

Lithium
Warfarin
Disulfiram
Propylene glycol (a food additive)
Cyclosporine
Lopinavir
Ritonavir

You should always tell your provider or pharmacist about all medications and supplements you are taking to avoid a drug interaction. Metronidazole won’t give you a UTI, but Dr. Marchand says it can cause dysuria, making it seem like your UTI is getting worse. He adds that it should be avoided with alcohol because mixing the two results in severe vomiting. It is contraindicated during the first trimester of pregnancy, and breastfeeding individuals should consult a provider before use.

Alternatives to metronidazole for UTI

Dr. Sylejmani says the efficacy of metronidazole for a UTI is limited to cases where anaerobic bacteria are involved, which is rare in uncomplicated UTI scenarios. “For UTIs, the bacteria most commonly responsible are aerobic gram-negative bacteria like E. coli,” he says.

The best antibiotics for UTIs are those targeting the typical aerobic bacteria responsible for the infection, including:

Fosfomycin is often noted for its single-dose efficacy, making it one of the quickest treatments for UTIs. Other, but less commonly prescribed, antibiotics used to treat UTIs include:

The bottom line? Metronidazole is rarely used to treat UTIs but can be effective against anaerobic bacteria like G. vaginalis, which causes BV. First-line antibiotics like nitrofurantoin and sulfamethoxazole-trimethoprim are typically the best options for typical UTIs.