Metronidazole Side Effects: What You Need to Know
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Metronidazole is an antibiotic commonly prescribed in the UK to treat various infections, including bacterial vaginosis (BV) and pelvic inflammatory disease. While generally considered safe and effective, it is important to be aware of its potential side effects.
Most people tolerate metronidazole well. Knowing what to expect — and what warning signs to watch for — helps you use the medication safely and confidently.
Common Side Effects
Gastrointestinal effects (very common)
The most frequently reported side effects of metronidazole affect the digestive system:
- Nausea and vomiting — most common, especially on an empty stomach
- Diarrhoea and stomach upset
- A metallic taste in the mouth (very characteristic of metronidazole)
- Furry or coated tongue
These side effects are usually mild and subside as your body adjusts to the medication. See managing tips in the section below.
Other common effects
- Dizziness or lightheadedness
- Headache
- Loss of appetite
Serious Side Effects
Although rare, serious side effects can occur. If you experience any of the following, seek immediate medical attention:
Liver Problems
Yellowing of the skin or eyes (jaundice), dark urine, or persistent upper abdominal pain may indicate liver issues.
Blood Disorders
Unexpected infections, mouth ulcers, easy bruising or bleeding gums, or extreme tiredness may suggest a blood problem.
Pancreatitis
Severe stomach pain that radiates to the back — this requires urgent medical evaluation.
Nervous System Effects
Blurred or double vision, seizures, mood changes, or numbness and tingling in the hands and feet (peripheral neuropathy).
Signs of meningitis (seek emergency care — call 999): High temperature, stiff neck, confusion, hallucinations, sensitivity to light, and difficulty speaking. These rare but serious symptoms require immediate medical attention.
Severe Allergic Reactions (Anaphylaxis)
Rarely, metronidazole can cause a severe allergic reaction (anaphylaxis). This is a medical emergency. Call 999 immediately if you experience:
- Skin rash — itchy, red, swollen, blistered, or peeling skin
- Wheezing or difficulty breathing
- Tightness in the chest or throat
- Swelling of the mouth, face, lips, tongue, or throat
Metronidazole and Alcohol — A Critical Warning
It is crucial to avoid alcohol entirely while taking metronidazole and for at least 48 hours after finishing your course. This is one of the most important precautions associated with this medication.
Combining metronidazole with alcohol triggers a disulfiram-like reaction — so named because it mimics the reaction caused by disulfiram (Antabuse), a medication used to treat alcohol dependence. This occurs because metronidazole inhibits the enzyme aldehyde dehydrogenase, causing acetaldehyde to accumulate in the blood. Symptoms include:
- Nausea and vomiting
- Stomach pain and cramps
- Hot flushes and sweating
- A pounding heartbeat (palpitations)
- Headache and dizziness
This reaction can occur even with small amounts of alcohol — including in certain foods, sauces, and mouthwashes. Check ingredient labels carefully during treatment.
Drug Interactions
Inform your prescriber about all medications you are taking before starting metronidazole — including over-the-counter medicines and herbal remedies. Key interactions include:
| Medication | Interaction |
|---|---|
| Warfarin | Metronidazole may increase the anticoagulant effect of warfarin, raising the risk of bleeding. INR monitoring may be required. |
| Lithium | Metronidazole can increase lithium blood levels, potentially causing lithium toxicity. |
| Disulfiram | Combining with disulfiram can cause psychotic reactions — avoid this combination. |
| 5-fluorouracil | Metronidazole may increase the toxicity of 5-fluorouracil (a chemotherapy drug). |
| Ciclosporin | May increase ciclosporin blood levels — monitoring required. |
Pregnancy and Breastfeeding
Metronidazole is generally considered safe during pregnancy and breastfeeding when prescribed by a qualified clinician — particularly for treating conditions such as BV, where untreated infection carries its own risks. However, you must always disclose pregnancy or breastfeeding status during your online consultation.
If you are breastfeeding and using metronidazole cream or gel, avoid applying it to the breasts to prevent the infant ingesting the medication. Your prescriber will advise on the appropriate formulation and course for your individual circumstances.
Tips for Managing Mild Side Effects
- Nausea and vomiting: Always take metronidazole after a meal or snack. Stick to simple, bland foods and avoid rich, spicy, or fatty foods during your course.
- Diarrhoea: Drink plenty of fluids to prevent dehydration. If diarrhoea persists beyond 24 hours, contact your prescriber.
- Metallic taste / furry tongue: Drink plenty of water and eat plain foods. Brush your teeth regularly. If you develop a white coating on your tongue, consult your prescriber about possible oral thrush.
- Headache: Stay well hydrated. A mild painkiller such as paracetamol may help — always check for drug interactions first.
Complete the full course: Always finish the prescribed course of metronidazole, even if your symptoms improve before the end of treatment. Stopping early may allow the infection to return and increases the risk of antibiotic resistance.
Antibiotic Resistance
Like all antibiotics, overuse or misuse of metronidazole can contribute to antibiotic resistance — where bacteria evolve to survive exposure to the antibiotic, making it less effective. To minimise this risk: only take metronidazole when prescribed; always complete the full course; and do not share your medication or use leftover antibiotics.
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Treat Bacterial Vaginosis Online →Frequently Asked Questions About Metronidazole Side Effects
What are the most common side effects of metronidazole?
The most common side effects include nausea, vomiting, diarrhoea, stomach upset, and a metallic taste in the mouth. Some people experience dizziness or headaches. These are usually mild and subside as your body adjusts to the medication.
Can I drink alcohol while taking metronidazole?
No — avoid alcohol entirely during treatment and for at least 48 hours after finishing your course. Combining metronidazole with alcohol triggers a disulfiram-like reaction causing nausea, vomiting, flushing, and palpitations. Check food and mouthwash ingredients too.
Does metronidazole interact with warfarin?
Yes — metronidazole may increase the anticoagulant effect of warfarin, raising the risk of bleeding. Always inform your prescriber about all medications you are taking, including blood thinners.
Is metronidazole safe during pregnancy?
Metronidazole is generally considered safe during pregnancy and breastfeeding when prescribed by a qualified clinician. Always disclose your pregnancy or breastfeeding status during your consultation so your prescriber can advise appropriately.
What should I do if metronidazole causes nausea?
Take metronidazole after a meal or snack. Stick to simple, bland foods and avoid rich or spicy food during your course. If nausea is severe or persistent, contact your prescriber — they may be able to advise on timing or alternative formulations.
When should I seek urgent medical help while taking metronidazole?
Seek immediate help if you experience jaundice, severe abdominal pain, unusual bruising or bleeding, vision changes, seizures, numbness or tingling in the extremities, a stiff neck with high temperature, or signs of severe allergic reaction. Call 999 in an emergency.
References
- NICE. Bacterial vaginosis. Clinical Knowledge Summaries. Available at: cks.nice.org.uk/topics/bacterial-vaginosis
- NHS. Bacterial vaginosis. Available at: nhs.uk/conditions/bacterial-vaginosis
- Electronic Medicines Compendium. Metronidazole tablets 400mg — SmPC. Available at: medicines.org.uk/emc
- Electronic Medicines Compendium. Dalacin Cream 2% w/w — SmPC. Available at: medicines.org.uk/emc
- GPhC. Standards for registered pharmacies. Available at: pharmacyregulation.org
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. Metronidazole and Dalacin Cream are prescription-only medicines — a medical consultation is required before they can be dispensed. In a medical emergency, call 999.