The association between market availability and adherence to antihypertensive medications: an observational study.

BACKGROUND High adherence to angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) reported in observational studies has frequently been attributed to improved tolerability. However, these agents are also relatively new to the market compared to other antihypertensive medications. We aimed to determine if an association exists between adherence and market availability of a specific antihypertensive agent. METHODS This retrospective cohort study used administrative data from Saskatchewan, Canada. Subjects were ≥40 years of age and received a new antihypertensive medication between 1994 and 2002. The primary outcome was the proportion of subjects achieving optimal adherence (≥80%) at 1 year, stratified by antihypertensive medication class and the year of availability. Adherence was measured using the cumulative mean gap ratio. RESULTS A total of 36,214 subjects met the inclusion criteria. Optimal adherence was observed in 4987 of 8623 (57.8%) subjects receiving ACEIs and 1013 of 1600 (63.3%) subjects receiving ARBs, but adherence appeared inconsistent when examined within each antihypertensive class. A pattern of increasing mean adherence was observed according to availability in the ACEI subgroup (Spearman r = 0.82; P = 0.007) but not the ARB subgroup (Spearman r = 0.41; P = 0.49). However, the association between availability and optimal adherence converged when ARB and ACEI users were combined (Spearman r = 0.85, P < 0.001). CONCLUSIONS Optimal adherence with ACEIs and ARBs compared to other antihypertensive agents may be associated with their relative availability. To what extent optimal adherence is also associated with improved tolerability, as currently believed, remains to be determined.