# Beta-3 Adrenergic Agonists > Beta-3 agonists in MBBS Pharmacology: mirabegron and vibegron mechanisms in overactive bladder, dosing, hypertension caution and paediatric intravesical use. - Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/beta-3-agonists - Exam / course: MBBS · Subject: Pharmacology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Beta-3 Adrenergic Agonists", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/beta-3-agonists ## Direct answer Mirabegron (25-50 mg daily) and vibegron (75 mg daily) are beta-3 selective agonists that relax the detrusor during the storage phase — beta-3 receptors constitute about 95 percent of bladder adrenoceptors, and their cyclic-AMP-mediated relaxation increases functional capacity without the dry mouth, constipation and cognitive cloud of antimuscarinics. They are first-line alternatives in overactive bladder when anticholinergic effects are intolerable, in the elderly at risk of delirium, and can combine with antimuscarinics for refractory symptoms. Their signature cautions are dose-related blood pressure rise and interactions via CYP2D6 (mirabegron), with intravesical mirabegron an emerging paediatric option for neurogenic detrusor overactivity in spina bifida children. ## What you must remember - **Mechanism map:** beta-1 heart, beta-2 bronchi and vessels, beta-3 bladder, adipose tissue and (weakly) gut — detrusor relaxation during filling is beta-3's physiological job, so pharmacology imitates storage-phase physiology. - **Mirabegron dosing:** 25 mg once daily, titrating to 50 mg after 4-8 weeks; reduce to 25 mg with strong CYP3A4 inhibitors, severe renal or hepatic impairment; take with or without food. - **Vibegron:** 75 mg once daily regardless of meals, cleaner blood-pressure profile and fewer CYP2D6 interactions in trials — the newer alternative where available. - **Blood pressure discipline:** mirabegron 50 mg raises systolic pressure by 1-2 mmHg on average — check pressure before and during therapy, and avoid in severe uncontrolled hypertension. - **Combination logic:** mirabegron plus an antimuscarinic (for example solifenacin) outperforms either alone in trials — dual mechanism: block contractile muscarinic drive and enhance beta-3 relaxation. - **Filling the anticholinergic gap:** no dry mouth, no constipation, no accommodation failure, no confusion — decisive in elderly patients and those on other anticholinergic drugs. - **Paediatric frontier:** intravesical mirabegron has been studied in children with neurogenic detrusor overactivity (spina bifida), reducing detrusor pressures where oral antimuscarinics fail — an off-label, specialist-use fact worth hedging as emerging practice. - **Other beta-3 biology:** brown-adipose thermogenesis and gallbladder relaxation — the reason beta-3 agonism was once pursued as an anti-obesity target before the bladder won. ## A clinic pathway worked through A 68-year-old woman with urgency, frequency and two-hourly nocturia has already abandoned oxybutynin for dry mouth and constipation; her MMSE is normal but she lives alone and her daughter worries about "confusion tablets". Urodynamic-minded history and a voiding diary confirm overactive bladder; urinalysis excludes infection and glucose; post-void residual is 60 mL. The switch is to mirabegron 25 mg each morning, with blood pressure checked at baseline and after a month and an explicit explanation that benefit accrues over 4-8 weeks. At review, urgency has halved, mouth is comfortable, and systolic pressure is up 4 mmHg — acceptable, monitored. If she had remained symptomatic, options would be vibegron 75 mg (if available), combination with solifenacin, or — before invasive steps — posterior tibial nerve stimulation and, in refractory cases, intradetrusor botulinum toxin with its voiding-dysfunction trade-off. A second vignette anchors the paediatric angle: a spina bifida child on clean intermittent catheterisation with high-pressure bladder despite oral antimuscarinics — specialist centres have used intravesical mirabegron instillations through the catheter to lower detrusor pressures, an evolving, off-label practice. ## How the exam frames it The receptor-distribution question leads: what percentage of bladder adrenoceptors are beta-3 (about 95 percent), and what does each subtype do to the bladder — beta-2 and beta-3 relax detrusor, alpha-1 contracts the bladder base and prostate (the tamsulosin connection). The second framing is comparative: antimuscarinics versus beta-3 agonists in overactive bladder — onset, side-effect profile, cognition, pressure — with the exam-ready aphorism that antimuscarinics block the "go" signal while beta-3 agonists strengthen the "hold" signal. The third is interaction pharmacology: mirabegron inhibits CYP2D6 (raising metoprolol and desipramine exposure) and is itself a 3A4 substrate — the kind of detail separating a distinction script from a pass script. A dark-horse question is why beta-3 agonists do not treat asthma like beta-2 agents: selectivity ratios and airway receptor density, not just "different receptor". ## Frequently asked questions ### How do beta-3 agonists relieve overactive bladder? Stimulation of beta-3 receptors (about 95 percent of bladder adrenoceptors) raises cyclic AMP in detrusor smooth muscle, relaxing it during storage and increasing capacity. ### What is the standard dose of mirabegron? 25 mg once daily, increased to 50 mg after 4-8 weeks if needed; 25 mg maximum with strong CYP3A4 inhibitors and in severe renal or hepatic impairment. ### Which patients benefit most from a beta-3 agonist over an antimuscarinic? Those with intolerable dry mouth or constipation, the elderly at risk of cognitive effects, and patients already burdened with anticholinergic drugs. ### Does mirabegron raise blood pressure? Yes, by about 1-2 mmHg systolic at 50 mg — blood pressure is checked before and during treatment, and uncontrolled severe hypertension is a contraindication. ### Can beta-3 agonists be combined with antimuscarinics? Yes — combination therapy (for example mirabegron with solifenacin) improves symptoms over either drug alone in trials.