Oxybutynin is a
muscarinic receptor antagonist, which has been available for a number of years in its original immediate-release (IR) formulation. While
oxybutynin IR has proven effective for the treatment of
overactive bladder, its extended use can be limited by adverse effects, particularly dry mouth. An extended-release (ER) formulation of
oxybutynin based on the OROS system has recently become available, which allows once daily administration. In direct comparison to
oxybutynin IR,
oxybutynin ER has an increased oral bioavailability for the parent compound
oxybutynin which is accompanied by a reduced bioavailability for the active metabolite N-desethyl-
oxybutynin. The latter has been implicated in mediating a major part of the adverse effects of
oxybutynin treatment. Two double-blind, placebo-controlled, randomised studies in patients with
overactive bladder have demonstrated that
oxybutynin ER has a similar efficacy as
oxybutynin IR but with improved tolerability. This is in line with clinical pharmacological studies demonstrating a smaller impairment of saliva production with
oxybutynin ER than with
oxybutynin IR. Thus, the ER formulation of
oxybutynin maintains the therapeutic benefits and concomitantly improves tolerability.