Mirabegron (Betmiga): overactive bladder medicine

Mirabegron eases the symptoms of overactive bladder: a sudden urgent need to pass urine, needing to go more often than usual, and leaking if you cannot reach a toilet in time. The NHS is clear that it does not treat the underlying condition.

It is a prescription-only slow-release tablet, sold as Betmiga, designed to release the medicine slowly and evenly through the day.

Mirabegron is a beta-3-adrenergic-receptor agonist. It relaxes the bladder muscle so the bladder can hold more and you do not need to pass urine as often or as urgently.

This is a different route from antimuscarinic (anticholinergic) tablets such as solifenacin and oxybutynin, which block muscarinic receptors. The NHS overview names urinary tract infections and a fast heart rate as typical reactions to mirabegron, and its detailed side effects page also lists nausea, constipation, diarrhoea, headache and dizziness.

The NHS describes 25 mg and 50 mg slow-release tablets, with 50 mg once daily the standard and 25 mg once daily for people with kidney or liver problems. Take it at the same time each day, with or without food, swallowing the tablet whole with water and never chewing or crushing it.

Treatment is usually long-term. Do not stop without medical advice, as symptoms may return or worsen, although a doctor may advise stopping if your bladder problems settle.

• If you remember a missed dose, take it unless the next dose is within 6 hours; then skip it. Never double up.

• An extra dose is unlikely to harm, though it can cause a faster heartbeat, headache and dizziness.

• Contact NHS 111 if you feel unwell after extra doses, and go to A&E if needed without driving yourself.

Most adults aged 18 and over can take it. Tell your doctor if you have had an allergic reaction to mirabegron or other medicines, liver or kidney problems, high blood pressure, urinary retention or a bladder blockage, long QT syndrome (a heart rhythm condition), or if you are pregnant, planning pregnancy or breastfeeding.

Usually mild effects include nausea, constipation, diarrhoea, headache and dizziness, and a fast heartbeat. If your heart rate rises, lie down and relax and call NHS 111 if it does not slow with rest. Taking mirabegron with food can ease nausea.

Call NHS 111 or see a doctor for trouble emptying the bladder or starting to pass urine, pain or burning on urinating, irritated or swollen eyes or eyelids, small non-itchy purple spots on the skin, or a fast heartbeat that persists. Call 999 or go to A&E for chest pain, a very severe headache or difficulty breathing, and for signs of serious allergy such as swollen lips, mouth, throat or tongue.

Suspected side effects can be reported to the Yellow Card scheme.

Mirabegron is not usually recommended in pregnancy because of limited safety data, and people with troublesome symptoms should talk to a doctor about alternatives. It may be used while breastfeeding if the baby is healthy, although better-studied medicines might be preferred. There is no evidence it reduces fertility.

The NHS names digoxin, imipramine, dabigatran, clarithromycin, ketoconazole, itraconazole and HIV medicines such as ritonavir. Paracetamol and ibuprofen are safe with it. Most herbal remedies have not been tested for interactions.

Alcohol does not change how mirabegron works, but it can worsen urgency, frequency and sleep. You can usually drive, but not if you feel dizzy. Drinks containing caffeine, alcohol, fizz, artificial sweeteners or citrus juice may aggravate bladder symptoms, so cutting back can help.

Mirabegron is generally safe to take long term as long as side effects are not troublesome, and many people use it for months or years.