Background: Adherence to statin treatment is expected to be associated with health outcomes.  Much of the available evidence is derived from studies conducted on selected populations (e.g. Medicaid population), on specific cohorts of patients (e.g. patients with diabetes mellitus or those who have experienced acute myocardial infarction (AMI)), or with respect to a single outcome (e.g. only death or only AMI).

Objective: The aim of this study was to evaluate the association between adherence to statin therapy and all-cause mortality and cardiovascular morbidity (AMI and stroke) in an unselected cohort of newly treated patients.

Methods: We performed a population-based retrospective cohort study that included adult patients with a first prescription of a statin from 1 Jan 2004 to 31 Dec 2006, using data from the administrative databases of the Local Health Unit of Florence, Italy.  Adherence to statin treatment was estimated as the proportion of days covered (PDC) by filled prescriptions and classified as low (PDC, 21%-40%), intermediate-low (PDC, 41%-60%), intermediate-high (PDC, 61%-80%), and high (PDC, over 80%).  Cases with PDC of 20% or lower were excluded.  A Cox regression model was used to investigate the association between adherence to treatment and all-cause mortality and hospitalisation for AMI or stroke.

Results: The cohort consisted of 19,232 patients (9823 men and 9409 women) aged 18 to 102 years (mean (SD), 66.5 (11.4) years): 20.1% had been previously hospitalised for cardiovascular events and 17.6% had been treated with hypoglycaemic drugs.  Adherence to statins was low in 4427 patients (23.0%), intermediate-low in 3117 (16.2%), intermediate-high in 3784 (19.7%) and high in 7904 (41.1%).  Lower-adherent patients were younger and had fewer comorbidities compared with higher-adherent patients.  In our multivariable analyses, high adherence was significantly associated with decreased risk of all-cause death, AMI, or stroke.  Compared with low adherence (hazard ratio (HR) = 1), the risk was lower in intermediate-low adherence (HR = 0.83; 95% CI, 0.71 to 0.98; P less than 0.05) and much lower in intermediate-high (HR = 0.60; 95% CI, 0.51 to 0.70; P less than 0.001) and high adherence (HR = 0.61; 95% CI, 0.54 to 0.71; P less than 0.001).

Conclusions: In this Italian cohort of newly treated patients, suboptimal adherence to statins occurred in a substantial proportion of patients and was associated with increased risk of adverse health outcomes.