Metronidazole: uses, the alcohol rule and side effects

The NHS lists skin infections, rosacea, mouth infections such as infected gums and dental abscesses, bacterial vaginosis, pelvic inflammatory disease, infected insect bites, skin ulcers, bedsores and wounds, and the treatment or prevention of certain bacterial and parasitic infections.

Because it comes in so many forms, which one you receive matters. Tablets, liquid and suppositories act through the body, a vaginal gel works locally, and skin gel or cream is used on the skin for rosacea or infected skin. The NHS says it works by stopping the growth of the bacteria or parasites causing the infection.

Metronidazole has one instruction that stands out. The NHS says not to drink alcohol while taking tablets or liquid or using suppositories, and to keep avoiding it for 2 days after the course ends. Mixing them can cause nausea, stomach pain, hot flushes, palpitations and headaches.

Some liquid medicines and herbal supplements contain alcohol, so check labels. Topical gel and cream have little effect on the rest of the body.

Oral treatment ranges from a single dose to a course of about 2 weeks at two or three times a day. Swallow tablets whole with water, with or just after food. Liquid has no food requirement. Vaginal gel is one applicator at night for 5 nights, and skin cream or gel is used twice a day for around 2 months for rosacea.

Complete the whole course even if you feel better, because stopping early risks the infection returning. Rosacea may take weeks to improve.

• Missed dose: take it as soon as you remember unless the next is near; never double up.

• Wipe or wash off excess cream or gel. Call 999 if a large amount of cream or gel is swallowed.

• If you are no better 2 to 3 days after the course, or get worse, tell your doctor.

Most adults and children can take metronidazole. The NHS asks about earlier allergic reactions, liver problems, nervous system disease, dialysis, and pregnancy or breastfeeding. It can interfere with some tests, so mention it before blood work.

Anyone using vaginal gel should report possible thrush and mention if a period is due.

Common effects are nausea, vomiting, diarrhoea, a metallic taste and a furry tongue. Drinking plenty of water and eating plain foods helps. Skin cream or gel can irritate, in which case use it less often or pause.

• Call NHS 111 or a doctor for yellow skin or eyes, unexpected infections, mouth ulcers or extreme tiredness, severe stomach pain spreading to the back, or blurred or double vision.

• Call 999 for headache with high temperature and stiff neck, confusion or hallucinations, or anaphylaxis.

The NHS names warfarin, lithium, disulfiram, phenytoin and phenobarbitone, ciclosporin, fluorouracil and busulfan, and liquid medicines containing alcohol. This matters mainly for tablets, suppositories, liquid and vaginal gel; skin products are absorbed too little to be a concern.

You may be able to use metronidazole in pregnancy; speak to your doctor if you are pregnant or trying. It passes into breast milk in modest amounts and a few babies have had diarrhoea; wash your hands after applying cream or gel before handling your baby.

Metronidazole does not stop contraception working, apart from the effect of severe vomiting or diarrhoea lasting over 24 hours. Avoid driving if you have confusion, hallucinations, drowsiness or blurred vision. Suspected side effects can be reported through Yellow Card.