- Reference pricing (RP) limits drug plan reimbursement of interchangeable medicines to a reference price, which is typically equal to the price of the lowest-cost interchangeable drug; any cost above that is borne by the patient. Much of the evidence of the effects of RP comes from 'before and after' studies of the RP scheme adopted by Pharmacare, the publicly funded drug plan for seniors and others in British Columbia, Canada. We critically assess the identifying assumption inherent in the before and after design - namely, that pre-RP trends accurately predict counterfactual outcomes - in the context of estimating the impact of RP on Pharmacare's expenditure on anti-hypertensive drugs for its senior beneficiaries. We use similar data from a public plan that has not introduced RP to estimate the effects on drug expenditures of patent expiration, secular changes in prescribing patterns and various other factors common to all Canadian public drug plans that could potentially confound the before and after estimates of the effect of RP on drug plan expenditures. We find that controlling for such factors reduces estimates of drug plan savings attributable to RP of the Calcium Channel Blockers by about half.