Disparities in initial oral antidiabetic medication adherence among veterans with incident diabetes

BACKGROUND: Medication nonadherence is a prevalent public health issue, particularly among patients with diabetes mellitus (DM), and negatively affects health outcomes. Because of the prevalence of DM among U.S. veterans, it is crucial to understand how well these patients adhere to oral antidiabetic (OAD) medication and whether certain subgroups are more likely to be nonadherent. OBJECTIVE: To assess initial OAD medication use among veterans with uncomplicated DM and determine factors associated with adherence in the first 2 years of treatment. METHODS: This was a retrospective cohort study using data from the Veterans Affairs (VA) Corporate Data Warehouse from 2002 through 2014. The first diagnosis for uncomplicated DM was determined, and then medication use was assessed following OAD initiation. OAD use was assessed by proportion of days covered (PDC) for the first 2 years of therapy using outpatient VA pharmacy records. Adherence was determined both continuously and categorically, with a PDC of ≥ 80% used to indicate adherence. Logistic regression was used to determine if certain patient characteristics were associated with being adherent to OADs. RESULTS: A total of 148,544 veterans with uncomplicated DM were assessed, most of whom were white, aged ≥ 55 years, and initiated OAD therapy on metformin. A large portion resided in the southern part of the United States. In the first year, PDC averaged 79.2% (SD = 25.9), and 63.2% were adherent to OAD therapy; however, these numbers declined in the second year, when the average PDC was 71.3% (SD = 35.8), and only 59.1% were adherent. Over the course of both years, PDC averaged 75.3% (SD = 28.4), and 50.9% were adherent. The odds of being adherent were higher among older adults and significantly lower among veterans self-identifying as either African American (OR = 0.61; 95% CI = 0.59-0.63), Native American (OR = 0.67; 95% CI = 0.61-0.75), or Hawaiian/Pacific Islander (OR = 0.84; 95% CI = 0.76-0.92) when compared with whites. Veterans who were either divorced/separated (OR = 0.86; 95% CI = 0.83- 0.88) or never married (OR = 0.89; 95% CI = 0.86-0.93) also had lower odds of being initially adherent to OAD therapy compared with those who reported being married. Being nonadherent in year 1 was highly predictive of remaining nonadherent in year 2 (OR = 12.8; 95% CI = 12.23-12.94), with only 22.2% nonadherent in the first year (8.2% overall) becoming adherent in the second year of therapy. Across both years, all minorities were less likely to be adherent (compared with whites), and average adherence differed among all geographic regions of the country. CONCLUSIONS: Within the first year of OAD therapy, medication adherence was suboptimal among veterans with DM, and second-year results indicate that adherence is likely to decline over time. Future studies should consider deeper regional and subgroup analysis to determine what contributes to variation in medication use in communities across the country.