Early studies of extended-release oxybutynin in patients with overactive bladder used adjusted-dose regimens ranging from 5 to 30 mg/day to achieve an optimal balance of efficacy and tolerability. The safety and tolerability of extended-release oxybutynin at a fixed dose of 10 mg once daily (commonly prescribed in clinical practice) is reported using pooled data from 2 multicenter, randomized, double-blind, parallel-group trials with a similar study design. One study compared extended-release oxybutynin with immediate-release tolterodine 2 mg bid. The other study compared extended-release oxybutynin with extended-release tolterodine 4 mg qd. In total, 576 patients received extended-release oxybutynin, 399 received extended-release tolterodine, and 193 received immediate-release tolterodine. The incidence of adverse events (AEs) was similar in the three treatment groups (extended-release oxybutynin, 70%; extended-release tolterodine, 64%; and immediate-release tolterodine, 79%). The most common adverse event was dry mouth (extended-release oxybutynin, 29%; extended-release tolterodine, 22%; and immediate-release tolterodine, 33%). Other AEs occurring in more than 5% of patients in any treatment group included constipation, diarrhea, headache, urinary tract infection, pain, dyspepsia, and peripheral edema, with no apparent difference across treatment groups. Most AEs (>90%) were mild or moderate in intensity in all treatment groups. The proportion of patients who discontinued study medication due to AEs was 6.1, 4.8, and 7.8% in the extended-release oxybutynin, extended-release tolterodine, and immediate-release tolterodine groups, respectively. In total, 1.2, 1.0, and 1.6% of patients in the extended-release oxybutynin, extended-release tolterodine, and immediate-release tolterodine groups, respectively, discontinued study medication due to dry mouth.