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Purpose – The authors focus on understanding the relationship between costs and cost sharing on medication adherence for individuals who initiated a disease-modifying therapy (DMT) for the treatment of multiple sclerosis (MS). DMTs reduce the risk of relapse and are an essential component of MS treatment. Furthermore, the authors compare monthly payment levels for copayments versus coinsurance and estimate the effects on adherence.

Methods – Using the MarketScan Commercial Claims and Encounters database evidence from July 1 2005 to March 31 2008, the authors employ a multivariate two-stage least-squares model (2SLS) to examine the impact of copayments or coinsurance on the medication possession ratio (MPR).

Findings – Descriptive results show that the mean out-of-pocket (OOP) costs of DMT per month were higher for patients with coinsurance than for patients with copayments. For the cohort of patients with copayment there was little difference in monthly copayments across adherence thresholds. Regression analysis shows that an increase in cost sharing reduces adherence overall, but this effect was small and insignificant in the copayment cohort. In contrast, in the coinsurance cohort increased cost sharing was significantly associated with decreased adherence to DMT medication; with a 10% increase in cost sharing leading to an 8.6% decline in adherence.

Implications – Employers increasingly rely on coinsurance, despite evidence that reliance on coinsurance results in lower adherence. Our research findings suggest that coinsurance appears to be a greater obstacle to compliance, confirming predictions found in the theoretical literature.

Originality – This research converted counts of injectable treatments into a continuous adherence measure. Previous literature on cost sharing did not examine MS.