Medication adherence and use of generic drug therapies

OBJECTIVE: To assess whether lower copayments charged for generic drugs explain the improved drug adherence associated with use of generics. METHODS: We analyzed 2001-2004 healthcare claims data from 45 large employers. Study subjects were age \u3e or = 18 years, had 1 or more of 5 study conditions (hypercholesterolemia, hypertension, hypothyroidism, seizure disorders, type 2 diabetes), and new use of generic-only or brand-only drug therapy for that condition. We measured adherence as the medication possession ratio (MPR), and adequate adherence as MPR \u3e or = 80%. Logistic regressions were conducted to assess adequate adherence, adjusting for copayments. RESULTS: We identified 327,629 new users of drug therapy. The proportion starting generic therapies ranged from 9% (hypothyroidism) to 45% (hypertension). After 1 year, 66.2% of individuals with hypothyroidism achieved an MPR \u3e or = 80% compared with 53.4% with hypertension, 53.2% with hypercholesterolemia, 52.0% with diabetes, and 42.2% with seizure disorders. Generics were associated with greater adherence than brands in patients with hypercholesterolemia or diabetes (P \u3c.05). Lower adherence was seen in patients with hypertension or hypothyroidism (P \u3c.05). There was no difference in seizure disorders. The likelihood of achieving an MPR =80% with $0 copayments compared with $1 to $9 ranged from an adjusted odds ratio (AOR) of 1.32 for seizure disorders (95% confidence interval [CI] = 1.41, 1.43) to an AOR of 1.45 for hypothyroidism (95% CI = 1.43, 1.48). CONCLUSION: Generic prescribing was associated with modestly improved adherence in 2 of 5 study conditions. Copayments of $0 were associated with improved adherence across all conditions