Understanding prescription adherence: pharmacy claims data from the Contraceptive CHOICE Project

Background: We examined prescription adherence rates by contraceptive method among women who used oral contraceptive pills (OCP), transdermal patch or vaginal ring. Study Design: Women in the St. Louis area were provided their choice of OCP, patch or ring at no cost and followed for 18 months. Time between monthly refills was obtained from pharmacy data and analyzed as a marker of adherence. Risk factors for initial nonadherence were estimated using Cox proportional hazards; predictors for repeated nonadherence were analyzed using Poisson regression with robust error variance. Results: Overall, 619 participants filled 6435 contraceptive prescriptions with a median of 10 refills per participant. Only 30% of women (n=187) obtained all refills on time. In the time-to-failure analysis, use of vaginal ring and increased parity were predictors of early nonadherence (p<.05). In the multivariable analysis, use of the vaginal ring and history of abortion were risk factors for repeated nonadherence (p<.01). Conclusions: Even with financial barriers removed, pharmacy data show that many women inconsistently refill their contraception and may be at risk for unintended pregnancy. (C) 2011 Elsevier Inc. All rights reserved.