BACKGROUND: There is interest in reducing out-of-pocket payments for prescription medicines, but the effects of such interventions remain unclear. OBJECTIVE: To study the impact of changes to the public prescription drug insurance program in British Columbia (BC), Canada that eliminated copayments for low-income households. METHODS: We used administrative data from 2017 to 2021 from Population Data BC and a controlled interrupted time-series design to examine a 2019 policy that eliminated copayments for households with incomes below 3,750. Households with incomes over 5,000-who experienced no changes in public coverage-served as a control. Our primary outcomes were prescription drug expenditures and the number of prescriptions dispensed. We also conducted a pre-post analysis to study impacts on dispensing and expenditures across therapeutic classes. RESULTS: The intervention cohort included 9,095 patients representing 8,011 households with an average age of 48.4. The control cohort included 820,395 patients representing 471,778 households with an average age of 51.1. Copayment elimination led to a level increase of .85 (95 % CI : .13 - .03) in monthly drug expenditures and had no impact on the trend. The mean number of prescriptions dispensed had a level increase of 0.07 (95 % CI: 0.04 - 0.09) and the rate of dispensing increased by 0.006 prescriptions monthly (95 % CI: 0.002 - 0.010). Copayment elimination was associated with increased expenditures and dispensing across most therapeutic classes. INTERPRETATION: Copayment elimination for low-income households in BC led to significant increases in prescription drug expenditures and dispensing across drug classes. Eliminating copayments appears to be effective at improving access to medicines for lower-income families.