Background: Describing a therapeutic index for a drug is important for evaluating safe and effective dosage regimens. Therapeutic index can be evaluated as the relative position of the dose-efficacy and the dose-side effect curves. Oxybutynin XL (Ditropan XL), a once-daily oral controlled- release formulation for oxybutynin chloride, is being developed. Oxybutynin XL efficacy and side-effect data obtained from two parallel-group, randomized, controlled clinical trials were modeled to evaluate the therapeutic index. Methods: A nonlinear mixed-effects model was used to characterize the oxybutynin dose-efficacy and dose-dry mouth relationship. Weekly urge urinary incontinence episodes, the primary efficacy variable, is a discrete variable (counts) with only non-negative integer values and was therefore modeled as a Poisson variable. The probability of dry mouth severity (the most frequently reported side effect), assessed on a categorical four-point scale, was modeled with a proportional odds model. In the modeling process, it was assumed that the time effect was the same for the active and placebo treatments and that the drug effect was additive. Results and Conclusions: The urge urinary incontinence episodes declined log- linearly, and no significant difference was observed between the two formulations. However, there was a trend toward higher efficacy with oxybutynin XL than with immediate-release oxybutynin at the same dose in one study. Dose-dry mouth analysis showed that the probability of dry mouth with an increasing dose was significantly lower with oxybutynin XL than with immediate-release oxybutynin in the second study, and a similar trend was observed in the first study. By combining the dose-urge urinary incontinence and dose-dry mouth relationship, a wider therapeutic index was predicted for oxybutynin XL than for immediate-release oxybutynin.