The primary purpose of the study was to determine whether pen users would challenge the insufficient remaining dose (IRD) stop mechanism with sufficient force to affect the dose accuracy of the final dose. The secondary purpose was to determine the participant's positive and negative impressions of the Humalog/Humulin pen and the likelihood of using the new prefilled pen. Three different modifications to the prefilled pen's IRD stop feature were made. These three pen models then underwent environmental dose accuracy testing at various temperatures and humidities, and user dose accuracy testing by 64 patients with diabetes. Evaluation also involved challenging the IRD stop at various dialing torques. Thirty pens from each model were tested to failure of the IRD stop. A model of the prefilled pen was selected for commercialization that met the dose accuracy targets of +/- 1 unit (U) for insulin doses less than 20 U and +/- 5% of dose volume for doses equal to or over 20 U. The selected pen model was superior at the minimum (1 unit), median (30 unit) and maximum (60 unit) dose volumes. Also 92% (n = 59) of patients interviewed felt that the stop mechanism for the final dose was clear. Extensive testing in the development of a prefilled insulin delivery device demonstrates an accurate and reliable medical device.