Mirabegron interacts with many other drugs via
cytochrome P450 isoenzymes. It also has additive adverse effects, in particular
cardiac disorders, when combined with
antimuscarinic drugs. In view of animal data and the lack of clinical data,
mirabegron should not be used by women who are or may be pregnant. In practice, drugs have little value in treating urinary urgency attributed to "
overactive bladder". The risk of
adverse drug reactions is rarely justified, even when the disorder is severe.
Antimuscarinic disorders, such as dry mouth, are less frequent with
mirabegron than with
antimuscarinic drugs. Like
antimuscarinic drugs,
mirabegron can cause
cardiac arrhythmias, especially
tachycardia.
Mirabegron may also cause a dose-dependent increase in blood pressure. Other adverse effects include rare cases of
kidney stones and rare but sometimes serious skin reactions. When a treatable cause of urinary urgency with incontinence has been ruled out and non-
drug measures have failed, recourse to an
antimuscarinic drug is slightly effective but exposes patients to numerous, potentially severe adverse effects.
Mirabegron (Betmiga⁰, Astellas Pharma), a beta-3
adrenergic receptor agonist, is authorised for use in this setting in the European Union. Clinical evaluation of
mirabegron is mainly based on five randomised, double-blind trials versus
antimuscarinic drugs, lasting 3 to 12 months and including about 8000 patients with urinary urgency.
Mirabegron and the
antimuscarinic comparators were similarly effective, even after
antimuscarinic drug failure. A meta-analysis of four placebo-controlled trials including about 3500 patients suggested that
mirabegron was poorly effective: on average, treatment prevented one episode of
urinary incontinence every 2 days.