The purpose of this study was to estimate the incidence rate (IR) of 10 common cancers in new users of antimuscarinic overactive bladder (OAB) medications. We conducted a cohort study using data recorded in Danish registers for patients newly exposed to the OAB drugs, darifenacin, fesoterodine, oxybutynin, solifenacin, tolterodine or trospium in years 2004-2012, aged 18years and without cancer before treatment initiation. We estimated IRs for each study cancer (bladder, breast, colorectal, lung, melanoma, non-Hodgkin lymphoma, pancreas, prostate, renal and uterine), standardised by age and sex and explored IR trends over time since treatment initiation. For all cancer analyses, only the first incident targeted cancer was considered. Of 72,917 patients (60% women; mean age at treatment start: 66years), 3475 developed a study cancer during 259,072 person-years of follow-up. The most common study cancers were prostate (48.1% of study cancers in men), breast (40.0% of study cancers in women) and lung (15.4% of all study cancers). The overall standardised study cancer IR was 5.4 per 1000 person-years (95% confidence interval, 5.3-5.6); IRs were similar across individual OAB drugs. The standardised IRs for bladder and prostate cancers, which have symptoms in common with OAB, were highest in the first 6months of treatment initiation and lower thereafter. In contrast, IRs for other study cancers were nearly constant during follow-up. Cancer IRs did not vary substantially by individual OAB drug. Protopathic bias is a plausible explanation for the higher rates of bladder and prostate cancers observed soon after starting OAB drug treatment.
