Poor medication adherence remains highly prevalent and adversely affects health outcomes. Patients frequently describe properties of the pills themselves, like size and shape, as barriers, but this has not been evaluated objectively. We sought to determine the extent to which oral medication properties thought to be influential translate into lower objectively-measured adherence.

Retrospective cohort study.

US nationwide commercial claims database, 2016–2019.

Among patients initiating first-line hypertension, diabetes or hyperlipidaemia treatment based on clinical guidelines, we measured pill size, shape, colour and flavouring, number of pills/day and fixed-dose combination status as properties.

Outcomes included discontinuation after the first fill (ie, never filling again over a minimum of 1-year follow-up) and long-term non-adherence (1-year proportion of days covered

Across 604 323 patients, 14.6% discontinued after filling once (ie, were non-persistent), and 54.0% were non-adherent over 1-year follow-up. Large pill size was associated with non-adherence, except for thiazides (eg, metformin adjusted OR (aOR): 1.12, 95% CI: 1.06 to 1.18). Greater pill burden was associated with a higher risk of non-adherence across all classes (eg, metformin aOR: 1.58, 95% CI: 1.53 to 1.64 for two pills/day). Taking less than one pill/day was also associated with higher risk of non-adherence and non-persistence (eg, non-persistence statin aOR: 1.29, 95% CI: 1.20 to 1.38). Pill shape, colour, flavouring and combination status were associated with mixed effects across classes.

Pill burden and pill size are key properties affecting adherence for almost all classes; others, like size and combination, could modestly affect medication adherence. Clinical interventions could screen patients for potential intolerance to medication and potentially implement more convenient dosing schedules.