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 > 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 > 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 > 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 <.05). Lower adherence was seen in patients with hypertension or hypothyroidism (P <.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.