OBJECTIVES: To develop a method to adjust the medication possession ratio (MPR) for type 2 diabetes patients who used a vial or pen form of insulin. METHODS: Using a retrospective analysis of large US health claims data from 2003 to 2008, diabetes patients who had at least two insulin prescription fills during the pre-index period were selected. The index date was the date of the first fill of insulin during the identification period from 2004 to 2007. One year of pre- and post-index continuous enrollment was required. Patients were excluded from the study if they switched to another form of insulin during the post-index period or had a diagnosis of gestational diabetes during the pre- or post-index period. MPR was calculated as a measure of drug adherence for patients who used vial insulin ('Vial Cohort'), and for those who used pen form insulin ('Pen Cohort'). Since insulin is a multi-dose treatment and is available in several package sizes, traditional MPR calculation is not suitable for this study. We adjusted the MPR by multiplying the traditional MPR by (average days between prescription refills/average days' supply) for patients in both cohorts. RESULTS: The unadjusted MPR during the post-index period for patients who used the pen device is lower than for patients who used a vial (0.55 vs. 0.60, p=0.0082). After controlling for baseline patient characteristics as well as the differences in package size between the pen and vial insulin using the new calculation method, the adjusted MPR for patients in the 'Pen Cohort' was higher than for patients in the 'Vial cohort' (0.22 vs. 0.13, p=0.001). CONCLUSIONS: After modifying the traditional MPR by adjusting the package size of the pen or vial insulin device, the adjusted MPR showed that pen insulin users had a significant advantage in drug adherence over vial users.