A study of over 1,500 cancer patients showed that drug costs have a significant effect on whether patients stick to their treatment plan. The study’s subjects had been prescribed imatinib (Gleevec), a treatment for chronic myeloid leukemia, a type of blood cancer. Patients with higher co-payments were 42% more likely to skip doses and 70% percent more likely to stop taking Gleevec entirely. Missing only 15% of prescribed Gleevec doses significantly raises the chance of the cancer developing drug resistance and relapsing. The study also found drastic differences in out-of-pocket treatment costs, with co-payments ranging from nothing to $4,792 for a 30-day supply of Gleevec. The average co-payment amount more than doubled over the 9-year course of the study.