Estimating the efficacy of an interstitial cystitis/painful bladder syndrome medication in a randomized trial with both non-adherence and loss to follow-up.

We are motivated by a randomized clinical trial evaluating the efficacy of amitriptyline for the treatment of interstitial cystitis and painful bladder syndrome in treatment-naïve patients. In the trial, both the non-adherence rate and the rate of loss to follow-up are fairly high. To estimate the effect of the treatment received on the outcome, we use the generalized structural mean model (GSMM), originally proposed to deal with non-adherence, to adjust for both non-adherence and loss to follow-up. In the model, loss to follow-up is handled by weighting the estimation equations for GSMM with one over the probability of not being lost to follow-up, estimated using a logistic regression model. We re-analyzed the data from the trial and found a possible benefit of amitriptyline when administered at a high-dose level.