What is the mechanism of action of mirabegron and what is a common side effect?

• A. A muscarinic receptor antagonist that reduces bladder contractions; dry mouth.
• B. A beta-1 adrenergic agonist that increases bladder contractions.
• C. A PDE-5 inhibitor that relaxes smooth muscle.
• D. A beta-3 adrenergic agonist that relaxes the detrusor to increase capacity; common side effects include hypertension and tachycardia.

Answer: (D)

Mirabegron works by activating beta-3 adrenergic receptors in the bladder detrusor muscle. This receptor stimulation causes relaxation of the detrusor during the filling phase, increasing bladder capacity and reducing urgency and incontinence episodes. It isn’t an antimuscarinic that blocks acetylcholine receptors to reduce contractions, and it isn’t a PDE-5 inhibitor or a beta-1 agonist. The cardiovascular side effects—hypertension and tachycardia—stem from beta-adrenergic stimulation of the heart and blood vessels.