Purpose: Adherence measures the extent to which patients take medications as prescribed by their health care provider. The control of hypertension is dependent on medication adherence and may vary on the basis of antihypertensive medication class and other factors.
Methods: The Department of Veterans Affairs' automated pharmacy database captures pharmacy medication use; International Classification of Diseases, 9th Revision, diagnostic codes; and laboratory and patient demographic data on a monthly basis. Hypertensive patients who used thiazide diuretics, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, calcium channel antagonists, and alpha-blockers from July 2002 to December 2003 were studied. The first date of prescription filling for each patient within the date range was the index date from which fill and refill dates were collected for up to 18 months to calculate medication possession ratios and days out of medication ratios. Patients were categorized as adherent if the medication possession ratio was 80% or greater. Logistic regression was used to study the association of medication class, age, gender, ethnicity, Veterans Affairs facility, and co-diagnosis with diabetes, schizophrenia/psychosis, depression, and dementia with medication adherence.
Results: We studied 40,492 hypertensive patients taking at least one antihypertensive drug class. The average age per class ranged from 67.4 to 72.9 years; 96% were male; and 51% were white, 8% were African-American, 4% were Asian-American, and 3% were Hispanic. Unadjusted adherence rates based on the medication possession ratio ranged from 78.3% for thiazide diuretics to 83.6% for angiotensin receptor blockers (P<.001). The number of medications (either total or antihypertensive) and age were independent predictors of better adherence. Black ethnicity and depression were associated with worse adherence.
Conclusions: Adherence rates with all antihypertensive medications were high. Although there were statistical differences by drug class, these differences were small. Ethnicity and depression identified groups that might benefit from programs to improve adherence.