Mirabegron ๐
Related Subjects:
|Urinary Incontinence
|Antimuscarinic drugs
|Solifenacin
|Oxybutynin
๐ Mirabegron is a ฮฒ3-adrenoceptor agonist for overactive bladder. NICE TA290 recommends it when antimuscarinics are contraindicated, ineffective or not tolerated. It has no anticholinergic burden, which makes it useful in older people at risk of cognitive impairment.
๐ About
- Relaxes the detrusor during bladder filling โ โ bladder capacity, fewer urgency episodes.
- Takes up to 4โ8 weeks for full effect; review at 4 weeks and again by 3 months.
- Always check the BNF for the latest guidance.
โ๏ธ Mode of Action
- Stimulates ฮฒ3 receptors in detrusor smooth muscle โ relaxation via cAMP.
- Does not block muscarinic receptors, so avoids dry mouth, constipation and confusion.
๐ฏ Indications
- Overactive bladder with urgency, frequency and/or urge incontinence in adults, after bladder training (NICE NG123 in women; CG97 for lower urinary tract symptoms in men).
- Can be combined with an antimuscarinic (e.g. solifenacin) under specialist advice if monotherapy fails.
๐ Dose
| Situation | Dose |
|---|
| Standard adult dose | 50 mg once daily (modified-release) |
| eGFR 15โ29 or moderate hepatic impairment | 25 mg once daily |
| With strong CYP3A4 inhibitors + mildโmoderate renal or mild hepatic impairment | 25 mg once daily |
| eGFR <15 or severe hepatic impairment | Not recommended |
๐งช Monitoring
- Blood pressure before starting and regularly during treatment (MHRA 2015), especially in hypertensive patients.
- Symptom review at 4 weeks.
๐ Interactions
- Moderate CYP2D6 inhibitor โ โ levels of CYP2D6 substrates with narrow therapeutic index (e.g. flecainide, propafenone, metoprolol, some TCAs).
- Digoxin: levels rise - start digoxin at the lowest dose and monitor levels.
- Strong CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir): โ mirabegron exposure - see renal/hepatic dosing above.
- Caution with other QT-prolonging drugs.
โ ๏ธ Cautions
- Hypertension (BP may rise); history of QT prolongation.
- Bladder outlet obstruction or concurrent antimuscarinic use (urinary retention risk).
- Pregnancy and breastfeeding: avoid.
๐ซ Contraindications
- Severe uncontrolled hypertension (systolic โฅ180 mmHg and/or diastolic โฅ110 mmHg).
- Hypersensitivity.
๐ฅ Side Effects
- Common: UTI, tachycardia, headache, dizziness, nausea, constipation, diarrhoea.
- Uncommon: palpitations, atrial fibrillation, raised BP, dyspepsia, rash, pruritus, vaginal infection.
- Rare: hypertensive crisis, angio-oedema, urinary retention, eyelid oedema.
๐ Exam Pearls
- Preferred over antimuscarinics in frail older people because of anticholinergic burden (falls, delirium, cognitive decline).
- Check BP - uncontrolled severe hypertension is the key contraindication.
๐ References
