This paper is focused on modeling and performance investigation of insulin injection pens used by diabetic persons. A smart pen cap with two electrodes is used as a capacitive sensor for insulin dose detection and is presented in details. To faithfully analyze this medical device, both an analytical model and a fully parametrized three-dimensional physical model in ANSYS have been developed. The electrode capacitance is evaluated (by both simulations and analytical model), for the smart pen cap alone as well as for different insulin fill states in the insulin injection pen + smart pen cap complete system.