A retrospective cohort linked longitudinal medical and pharmacy claims to modelled individual-level behavioural and consumer attributes, via privacy-preserving tokenisation, for 401,450 US women who began tamoxifen or an aromatase inhibitor after breast surgery between 2016 and 2025. Adherence, defined as a medication possession ratio of 80% or more over the first 365 days, was achieved by 280,595 women (69.9%). Behavioural predictors remained significant after adjustment for clinical and demographic factors but were modest in size, including comfort following medication instructions (aOR 1.15, 95% CI 1.06-1.24) and proximity to a breast oncologist (aOR 1.17, 95% CI 1.04-1.32).
Why it is interesting: It sets a national first-year adherence figure for a therapy whose mortality benefit depends on five to ten years of daily use, and shows what a commercial behavioural-to-claims linkage can and cannot resolve.