The impact of the introduction of a formulary into a large Canadian private drug plan: an interrupted time-series analysis.

backgroundMost private drug plans in Canada do not use a formulary, which leads to suboptimal drug use. We studied the impact of the adoption of the public formulary by a large private health benefits plan in British Columbia.

methodsWe studied the impact of a change by members of the BC Hospital Employees' Union to have their private drug plan mirror the public formulary as of June 2013. With data from Pacific Blue Cross, we conducted a before-and-after descriptive study using interrupted time-series analysis to study changes in covered drug costs and use for 18 months preceding and following the change.

resultsOur cohort averaged 66 000 plan members and dependents over our study period. Following the implementation of the formulary, the number of prescriptions covered by the plan declined by 0.46 prescriptions per member per month (95% confidence interval -0.50 to -0.42), a decline of 23.8% at 1 year. This decreased plan spending by $1.32 million over the 18 months after the coverage change, a 49.7% decline.

interpretationThe adoption of the public formulary by a large private drug plan in BC substantially reduced drug plan expenditures and the volume of prescriptions paid for by the plan. Overall, these results suggest that carefully designed formulary changes could substantially reduce spending by private-sector drug plans on drugs that have more cost-effective therapeutic alternatives.