Metronidazole: Uses, Side Effects & Safety

What it is and what it's used for

Metronidazole is an antibiotic and anti-parasitic medicine (a nitroimidazole) used against infections caused by anaerobic bacteria and certain parasites. Doctors commonly prescribe it for amoebiasis (amoebic dysentery), giardiasis, bacterial vaginosis, trichomoniasis, dental and gum abscesses, infected wounds, and as part of combination treatment for Helicobacter pylori stomach infections. It is also used in hospitals, as an IV drip, for serious abdominal and pelvic infections.

In India, metronidazole is a prescription medicine (Schedule H) — it should only be used on a doctor's advice. Because it is widely used, it is also one of the medicines that should not be taken for ordinary stomach upsets without a proper diagnosis, since misuse contributes to drug resistance.

How it works (simple language)

Metronidazole works by damaging the DNA of the bacteria and parasites it targets. Once inside these organisms, the medicine is converted into a reactive form that breaks their genetic material, so they cannot survive or multiply. Human cells convert it in this way much less readily, which is why it acts mainly on the infection.

It is most effective against anaerobic organisms — those that live without oxygen — which is why it is used for gut, vaginal, dental, and deep-wound infections, but it does not work against ordinary viruses like the common cold or flu.

Dosage forms available in India (general info only)

Metronidazole is commonly available in India as 200 mg and 400 mg tablets, an oral suspension, a 500 mg/100 ml IV infusion for hospital use, and a 1% gel for skin and vaginal use. Well-known brand names include Flagyl and Metrogyl, though many generic versions are also available and brands vary by pharmacy.

Never decide the dose yourself from this article. Always follow your doctor's prescription or the directions on the pack, and check with your pharmacist if you are unsure — especially for children, where the dose depends on the child's weight and age.

Side effects

Common: Metallic or bitter taste in the mouth, nausea, stomach cramps or mild stomach upset, headache, dizziness, loss of appetite, and sometimes dark or reddish-brown urine (this is harmless and goes away after treatment).

Serious — seek medical care: Tingling, numbness, or burning in the hands or feet (signs of nerve damage — report it promptly), seizures or fits, severe allergic reactions (rash, itching, swelling of the face or lips, difficulty breathing), severe or bloody diarrhoea, or signs of a brain reaction such as confusion, unsteady walking, or difficulty speaking (rare).

Precautions and warnings

• No alcohol: Do not drink alcohol while taking metronidazole and for at least 48 hours after your last dose. Mixing them can cause severe nausea, vomiting, flushing, headache, and a racing heartbeat (a disulfiram-like reaction). This includes alcohol in some cough syrups and tonics — check labels.
• Nerve symptoms: Long courses or repeated use can cause tingling or numbness in the hands and feet. Tell your doctor at once if this happens; the dose may need to be changed.
• Liver disease: The liver breaks down metronidazole, so people with liver problems need a lower dose or closer monitoring. Tell your doctor about any liver condition.
• Finish the course: Stopping early because you feel better can let the infection return and may encourage drug resistance. Complete the full course exactly as prescribed.
• Allergies: Tell your doctor if you have ever had a bad reaction to metronidazole or to similar medicines. Do not take it again after a serious allergic reaction.
• Driving: Dizziness or drowsiness can occur. Avoid driving or operating machinery if you feel affected.
• Sunlight: Some people notice skin sensitivity; this is uncommon, but report any unusual rash.

Interactions (general)

Metronidazole interacts with alcohol (see above) and with the blood thinner warfarin — it can raise bleeding risk, so your doctor may check your clotting more often. It can also interact with lithium (used for bipolar disorder), raising lithium levels, and with some seizure medicines such as phenytoin and phenobarbital, which can make metronidazole less effective. Always tell your doctor about ALL medicines you take, including herbal and ayurvedic products, and mention alcohol use honestly.

Pregnancy and breastfeeding (general guidance)

Metronidazole is generally avoided during the first trimester of pregnancy unless the doctor decides the benefit clearly outweighs the risk; it may be used later in pregnancy when needed. Small amounts pass into breast milk, so a breastfeeding mother is usually advised to discuss timing or a brief pause in breastfeeding with her doctor. Always consult your doctor first if you are pregnant, planning pregnancy, or breastfeeding.

Storage

Store at room temperature, away from moisture and direct sunlight — keep the tablets in their original strip or container. The suspension should be shaken well before use and stored as directed on the label. Keep all medicines out of the reach of children, and do not use after the expiry date printed on the pack.

When to see a doctor

See a doctor if: your symptoms do not improve within the expected time (usually a few days), diarrhoea becomes severe or bloody, you develop tingling or numbness in the hands or feet, you get a rash or swelling, or you drank alcohol and develop severe vomiting or a racing heartbeat. Seek emergency care for signs of a severe allergic reaction (swelling of face/lips, difficulty breathing), seizures, confusion, or unsteady walking.

FAQs

1. Why does metronidazole cause a metallic taste in the mouth?

A metallic or bitter taste is one of the most common side effects of metronidazole and is harmless. It usually starts soon after the first doses and goes away once the course is finished. Sucking on a sugar-free mint or chewing gum can make it more bearable.

2. Can I drink alcohol while taking metronidazole?

No. Alcohol can trigger a severe reaction — intense nausea, vomiting, flushing, headache, and a racing heartbeat. Avoid alcohol during the course and for at least 48 hours after your last dose, and be careful with medicines or tonics that contain alcohol.

3. Why did my urine turn dark while on this medicine?

Metronidazole can turn urine dark or reddish-brown in some people. This is harmless and temporary — your urine returns to normal after you finish the course. However, if you also have pain while passing urine, fever, or back pain, see your doctor, as these may be signs of the infection itself.

4. Can I stop taking metronidazole once I feel better?

No — finish the full course exactly as your doctor prescribed. Stopping early can let the infection come back, sometimes stronger, and contributes to antibiotic resistance. If side effects are troubling you, talk to your doctor instead of stopping on your own.