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

Metronidazole (brand name Flagyl) is a highly effective antibiotic, but like all medications, it comes with side effects. Most people tolerate it well for short courses of treatment. However, some side effects — particularly neurological ones — can be serious and require prompt attention.

This guide covers what to expect when taking metronidazole, which side effects are worth monitoring, and the warning signs that mean you should call your doctor or seek emergency care immediately.

Common Metronidazole Side Effects (Affecting 1%+ of Users)

These are the side effects most people experience. They're usually mild and go away within a few days of finishing your course:

Metallic or bitter taste in the mouth: The most notorious and frequently reported side effect. It's caused by metronidazole being secreted in saliva and doesn't indicate a problem — but it's unpleasant. Some patients find it helps to use mint gum, hard candy, or brush teeth more frequently.

Nausea: Common, especially with the 2 g single dose used for trichomoniasis. Taking metronidazole with food or milk can reduce nausea.

Diarrhea: Occurs in some patients due to disruption of gut flora. Usually mild and transient.

Headache: A common complaint, usually mild. Staying well hydrated can help.

Dizziness: Some patients report lightheadedness. Avoid driving or operating heavy machinery until you know how metronidazole affects you.

Abdominal pain or cramping: Can occur, usually related to GI effects.

Vomiting: Less common than nausea. Taking metronidazole with food may help prevent this.

Vaginal yeast infection (candidiasis): Approximately 10% of women develop a yeast infection after metronidazole treatment because antibiotic use disrupts vaginal flora. Contact your doctor if you develop yeast infection symptoms after your course.

Dark urine: A harmless side effect — metronidazole metabolites can turn urine reddish-brown or dark. This is not blood and is not dangerous.

The Alcohol Interaction: Why You Must Avoid Alcohol on Metronidazole

This is one of the most important warnings associated with metronidazole. Do not drink alcohol while taking metronidazole — or for at least 3 days after your last dose. Combining alcohol with metronidazole can cause a disulfiram-like reaction, which includes:

Nausea and vomiting

Severe headache

Flushing and skin redness

Rapid heartbeat

Abdominal cramps and sweating

This reaction can last from 30 minutes to several hours. Also avoid foods and medications that contain propylene glycol (found in some processed foods and medications), as it causes a similar reaction.

Serious Side Effects: Stop Taking Metronidazole and Seek Care

The following side effects are rare but serious. Stop metronidazole immediately and seek medical attention if you experience:

Seizures (convulsions): Call 911 immediately. Seizures are a known but rare complication, more common with high doses or long-term use.

Encephalopathy (brain dysfunction): Signs include confusion, fever, headache, visual changes, weakness, hallucinations, or difficulty speaking. Stop the medication and seek emergency care.

Peripheral neuropathy: Numbness, tingling, or burning in the hands or feet — especially with prolonged use. Report to your doctor promptly. May be irreversible if not caught early.

Aseptic meningitis: Rare. Signs include severe headache, neck stiffness, fever, and sensitivity to light. Seek emergency care.

Severe skin reactions (Stevens-Johnson Syndrome, TEN): Very rare. Rash, blistering, skin peeling, especially around the face, eyes, or genitals. Stop the medication immediately and seek emergency care.

DRESS reaction: Drug Reaction with Eosinophilia and Systemic Symptoms — a severe multi-organ allergic reaction. Signs include widespread rash, fever, facial swelling, and organ involvement. Seek emergency care.

QT prolongation: Irregular heartbeat. Patients with pre-existing QT prolongation or those taking other QT-prolonging drugs should discuss this risk with their doctor before starting metronidazole.

Boxed Warning: Carcinogenicity Risk

Metronidazole carries an FDA boxed warning: it has been shown to cause cancer in laboratory animals (rats and mice). The FDA advises that metronidazole should be used only for approved indications and should not be used unnecessarily. This does not mean you should avoid it when clinically indicated — the benefits of treating your infection typically outweigh this theoretical risk for standard short-course treatment.

Tips to Reduce Common Side Effects

Take metronidazole with food or a glass of milk to reduce nausea and GI upset

Use sugar-free gum, mints, or hard candy to mask the metallic taste

Stay well hydrated throughout your course

Avoid alcohol and propylene glycol during treatment and for 3 days after your last dose

Report any neurological symptoms (numbness, tingling, confusion, seizure) to your doctor immediately

The Bottom Line

Metronidazole is generally safe for short-course treatment when used as directed. The most common side effects — metallic taste, nausea, and diarrhea — are annoying but not dangerous. The serious side effects (seizures, neuropathy, encephalopathy) are rare and primarily associated with high doses or prolonged therapy. The most important rule: no alcohol for the entire course plus 3 days after. See also our guide on

metronidazole drug interactions for a full list of what to avoid while taking this medication.