Oxybutynin acts as a competitive antagonist of acetylcholine at post-ganglionic muscarinic receptors, resulting in relaxation of bladder smooth muscle. Oxybutynin is a racemic (50:50) mixture of R- and S-isomers. Antimuscarinic activity resides predominantly in the R-isomer. The R-isomer of oxybutynin shows greater selectivity for the M1 and M3 muscarinic subtypes (predominant in bladder detrusor muscle and parotid gland) compared to the M2 subtype (predominant in cardiac tissue).