Clarithromycin: uses, side effects and interactions

Clarithromycin belongs to the macrolide group. The NHS lists chest infections such as pneumonia, the skin infection cellulitis and ear infections as typical uses, and it is also one of the antibiotics combined to treat Helicobacter pylori, the bacterium involved in some stomach ulcers.

A second reason it appears so often in prescribing is allergy. When a penicillin is not suitable, clarithromycin is an alternative for many of the same infections. The NHS compares it with azithromycin and erythromycin, and mentions that unlike some alternatives it can be used against H. pylori.

Standard tablets are taken morning and evening; the NHS gives a usual range of 250 mg to 500 mg twice a day. Slow-release tablets are taken once a day, typically 500 mg, and up to 1 g a day may be used for severe infections. Your label overrides any general figure.

Liquid is measured with the syringe or spoon that comes with it, and the bottle should be shaken well before each dose. Do not use a kitchen teaspoon. Complete the prescribed course, because the NHS warns that stopping early can allow the infection to return.

• Food: with or without; eat and drink as usual.

• Missed dose: take it as soon as you remember unless the next is nearly due, and never take two at once.

• Overdose: one extra dose is usually low risk; call NHS 111 if you have stomach pain, sickness or diarrhoea, and 999 if you become seriously unwell.

• Review: speak to your doctor if you are no better after 2 to 3 days or you develop new problems such as weight loss.

Most adults and children can take clarithromycin. The NHS asks you to tell the prescriber if you have had an allergic reaction to it or another medicine, or have had diarrhoea after antibiotics before.

The cardiac points matter most. Clarithromycin can affect heart rhythm, so mention an arrhythmia, any heart problem, or low blood potassium or magnesium. Other flags are myasthenia gravis, which it may make worse, liver or kidney problems, and pregnancy or breastfeeding. People with phenylketonuria should know that some liquids contain aspartame.

The NHS lists more than 1 in 100 people having nausea or vomiting, diarrhoea, bloating or indigestion, headaches and insomnia. Simple meals, taking the dose after eating and sipping water help the stomach, and a headache is usually managed with rest, fluids and limiting alcohol. For sleep, avoid large evening meals, smoking and caffeine.

Sickness or severe diarrhoea can reduce the contraceptive pill, so read its leaflet if either lasts more than a day.

• Call NHS 111 or a doctor for an irregular or rapid heartbeat, pale stools with dark urine and yellow eyes, severe stomach or back pain, bloody or mucus diarrhoea lasting more than 4 days, or hallucinations.

• Call 999 for facial swelling, difficulty breathing, throat tightness, a swollen rash, faintness or confusion.

Clarithromycin changes how the liver handles many other drugs, so the NHS list is long. It names carbamazepine and phenytoin for seizures, theophylline for asthma, colchicine for gout, digoxin, ivabradine and ranolazine for the heart, warfarin and ticagrelor, simvastatin and atorvastatin, midazolam and domperidone.

St John's wort is mentioned among herbal products, and the NHS says there is not enough information to call complementary medicines safe alongside it. Always give the prescriber and pharmacist a full list.

The NHS says clarithromycin is safe in pregnancy but other antibiotics may suit better, so tell the prescriber if you are pregnant or trying. It is considered safe for healthy breastfed babies; watch for poor feeding, diarrhoea, vomiting, rash or thrush, sleep disturbance, irritability or sweating.

It can cause dizziness. If it does, do not drive, cycle or use machinery until it passes, and avoid alcohol while dizzy. Otherwise alcohol is generally fine. It does not stop contraception working. Suspected side effects can be reported to the Yellow Card scheme.