OBJECTIVES: Elevating drug copayments has been found to negatively affect medication adherence among patients with chronic illness. This study sought to assess the correlation between prescription drug copayment and medication adherence upon initiating insulin aspart pen therapy from previous vial/syringe use among type 2 diabetes patients. METHODS: A pre-post conversion design was adopted using an integrated medical and pharmacy claims database from >50 managed care plans in the United States. Adults diagnosed with type 2 diabetes who converted to insulin aspart pen therapy (NovoLog® FlexPen®) [ie, the index event] from either human or analog insulin vials between July 2001 and December 2002 with no prior use of FlexPen® for six months were identified and retrospectively analyzed. Association between post-index medication adherence [medical possession ratio (MPR) = 80%] and drug copayment after adjusting for differences in baseline demographics, pre-index copayment, and clinical comorbidities and complications was analyzed through a logistic regression model. RESULTS: A total of 670 subjects were identified [mean age:45.7 years (SD:13.8 years)]. Drug copayments increased by approximately $2 ($13.2 vs. $11.0) after initiating FlexPen® compared to pre-index vial/syringe use. Logistic regression model results showed that individuals with higher drug copayments had a significantly lower medication adherence (MPR < 80%) as compared to individuals with lower copayments. Odds of adherence being = 80% decreased by 17% among individuals with $21 and above drug copayment [Odds Ratio: 0.83 (0.71-0.94); p<0.05], while the odds decreased by 10% among individuals with copayments ranging from $11-$20 [Odds Ratio: 0.90 (0.80-0.99); p<0.05], as compared to individuals with drug copayments = $10. CONCLUSION: Increased drug copayments were found to be a barrier to improved medication adherence among type 2 diabetes patients converting to insulin analog pen therapy. Implications of copayment variation on health outcomes in relation to medication adherence should be researched.