Metronidazole Side Effects: What to Expect and When to Worry

Part of the Access Doctor Bacterial Vaginosis guide.

Metronidazole is an antibiotic commonly prescribed in the UK for infections including bacterial vaginosis and dental infections. Most people tolerate it well — knowing what to expect, and which warning signs to watch for, helps you use it safely. For what it is and how to take it, see what metronidazole is and how to take it.

Common Side Effects

Gastrointestinal effects (very common)

• Nausea and vomiting — most common, especially on an empty stomach
• Diarrhoea and stomach upset
• A metallic taste in the mouth (very characteristic of metronidazole)
• A furry or coated tongue

Other common effects

• Dizziness or lightheadedness
• Headache
• Loss of appetite

These are usually mild and settle as your body adjusts — taking each dose with or after food reduces the nausea.

How Long Do Metronidazole Side Effects Last?

Mild side effects — nausea, metallic taste, headache — usually settle within a couple of days as you adjust, and clear within a few days of finishing the course. The metallic taste can linger briefly after the last dose. Anything that persists well after the course, or worsens during it, is worth a call to your prescriber.

Serious Side Effects

Although rare, serious side effects can occur. If you experience any of the following, seek immediate medical attention:

Severe Allergic Reactions (Anaphylaxis)

Rarely, metronidazole can cause a severe allergic reaction (anaphylaxis) — a medical emergency. Call 999 immediately if you experience:

• An itchy, red, swollen, blistered or peeling skin rash
• Wheezing or difficulty breathing
• Tightness in the chest or throat
• Swelling of the mouth, face, lips, tongue or throat

Metronidazole and Alcohol

Avoid alcohol entirely while taking metronidazole and for at least 48 hours after finishing your course. Combining them triggers a disulfiram-like reaction — metronidazole blocks the enzyme that breaks down acetaldehyde, which then builds up and causes nausea, vomiting, stomach cramps, hot flushes, palpitations and headache. It can happen with even small amounts, including alcohol in foods, sauces and mouthwashes, so check labels. For the practical side of dosing and timing, see how to take metronidazole.

Drug Interactions

Tell your prescriber about all medicines you take before starting metronidazole — including over-the-counter products and herbal remedies. Key interactions:

Pregnancy and Breastfeeding

Metronidazole is generally considered safe in pregnancy and breastfeeding when prescribed by a clinician — particularly for infections like BV, where leaving it untreated carries its own risks. Always disclose pregnancy or breastfeeding status during your consultation. If you're breastfeeding and using the cream or gel, avoid applying it to the breasts. Your prescriber will advise on the right formulation and course.

Tips for Managing Mild Side Effects

• Nausea: take metronidazole after a meal or snack; stick to bland foods and avoid rich, spicy or fatty foods during the course.
• Diarrhoea: drink plenty of fluids. If it persists beyond 24 hours, contact your prescriber.
• Metallic taste / furry tongue: drink water, eat plain foods, brush regularly. A white tongue coating may be oral thrush — ask your prescriber.
• Headache: stay hydrated; paracetamol may help (check interactions first).
• Finish the course: complete it even if you feel better, or the infection can return and resistance risk rises.