To the best of the authors' knowledge, this study is the first narrative systematic review on nonfulfillment of prescription medications. Despite the wide variation in individual study rates, the mean and median rates across different modes of data collection and sources of data were in a relatively narrow range (11% to 19%) and surprisingly close to the overall mean (16.4%) and median (15%.0) rates for all studies. The reasons for nonfulfillment identified through this review address barriers to nonfulfillment at the patient, physician, and health system level and thus bear important implications for policy makers.