OBJECTIVES: The impact of the type of drug insurance plan on costs of medications is not well known. We aimed to compare the costs of antidepressants and statins, two of the most prescribed drugs, in Québec patients who are covered by private and public drug insurance. METHODS: Two matched cohort studies were conducted using prescription claims databases for Quebec residents with private (reMed) and public drug insurance (RAMQ). Patients were aged 18 to 64 years and filled at least one prescription for an antidepressant (Cohort 1) or a statin (Cohort 2) in monotherapy between December 2007 and September 2009. The average monthly costs (medication cost + dispensing fee) of antidepressants and statins per patient and the percent cost difference for a 30-day prescription for each antidepressant and each statin product were compared between patients with private and public drug insurance using t-tests and regressionn models. RESULTS: Cohort 1 included 194 privately and 1923 publicly insured patients while the corresponding figures for Cohort 2 were 174 and 1712, respectively. The average cost of antidepressants and statins per patient per month was significantly higher among privately-insured than publicly-insured patients [Cohort 1: $48.17±25.19 vs. $33.72±17.58, p<0.001; Cohort 2: $61.19±22.31 vs. $54.62±20.48$, p<0.001]. The cost of 30-day prescriptions for most individual drugs of antidepressants and statins was significantly more expensive for privately-insured patients than publicly-insured patients. CONCLUSIONS: Higher cost of antidepressants and statins observed in privately-insured patients might be due to different pharmacy payment requirements and the dispensing fee restrictions under the public plan. More research is need to investigate whehter or not this higer drug cost translates into lower patient's adherence and persistence to medications.