BACKGROUND: For patients with a chronic disease, increased cost-sharing for medications may lead to unintended consequences, including reduced use of medications essential for control of their disease. OBJECTIVE: The objective of this study was to estimate the effects of small ($1-6 per 30-day supply), moderate ($7-10), and large (>$10) increases in medication cost-sharing on 12-month trends in oral hypoglycemic (OH) use among adults with type 2 diabetes. METHODS: We conducted a quasiexperimental study using a time series with comparison group design. Data were obtained from computerized membership, benefit, and pharmacy dispensing data of 5 managed care organizations (MCOs). A total of 13,110 12-month episodes of OH use and a medication cost-sharing increase ("intervention") were matched with 13,110 that had no increase. The dependent variable was OH average daily dose (ADD) standardized to each episode's mean OH ADD in the 6-month preintervention period. The principal independent variable was change in cost per 30-day OH supply between the 6-month pre- and postintervention periods. Effects of changes in cost-sharing on OH ADD were estimated using segmented time series regression. RESULTS: Episodes with >$10 increase in cost-sharing had significantly (alpha=0.05) decreased OH ADD in the postintervention period. At 6 months after this increase, OH ADD had decreased by 18.5% from that predicted from the preintervention trend. Episodes with a $1 to $10 increase in cost-sharing and those with no increase in cost-sharing had significant linear increases in OH use over the 12-month period. CONCLUSIONS: Large increases in medication cost-sharing were associated with immediate and persistent reductions in OH use. Small and moderate increases had little effect on OH use in the 6-month period after the increase.