OBJECTIVES: To compare the discontinuation rates due to poor tolerability in patients with overactive bladder treated with either once daily dosed extended release (XL), or twice or greater daily dosed, immediate release (IR) oxybutynin. METHODS: We used the real-world longitudinal data (1995-2002) from IMS Mediplus UK to identify all patients new to therapy initiated on either oxybutynin IR or XL. The first script defined the patients study cohort and the date of that script was the study index date from which monitoring outcomes followed. These scripts were linked to reason to stop therapy, including poor tolerability. The start-date-match approach was used to adjust for the later market entrance of oxybutynin XL. We used c2 test to evaluate the statistical significance in the difference of discontinuation rates between both groups. RESULTS: We identified 147 patients initiated on oxybutynin XL and 216 patients on oxybutynin IR, with an average daily dose of 10.0 mg and 8.9 mg respectively. Patient demographics were comparable. In the oxybutynin XL group, 29 patients (19.7%) had a record for stopping therapy compared to 129 patients (67.6%) in the oxybutynin IR group. Of those with a stopping therapy record, discontinuation rates due to poor tolerability were 3.4% versus 14.4% (p <0.01) for the XL and IR groups, respectively. The most frequent recorded reason for the oxybutynin IR group was change in formulation or titration, 20.8% compared to 4.8% for the XL group, followed by poor tolerability, and lack of effect (8.3% for the IR group compared to 3.4% for the XL group). Of the 216 oxybutynin IR patients, 120 subsequently switched to oxybutynin XL. CONCLUSIONS: The improved tolerability of oxybutynin XL compared to oxybutynin IR demonstrated in randomized clinical trials is reflected in the UK real-world clinical practice setting: patients on oxybutynin XL have significantly lower discontinuation rates.