BACKGROUND: Previous studies have concluded that there is significant variation in drug coverage across Canadian provinces because conventional measures of inter-rater reliability for formulary listings are low. We sought to investigate whether conventional methods are appropriate for formulary concordance measurement by testing the hypotheses that (a) conventionally measured variations in provincial formularies are driven by disagreement over large numbers of drugs that represent very small segments of the market and (b) patterns in coverage levels and agreement across therapeutic categories might provide evidence of "potentially legitimate" variation in provincial formularies. METHODS: We studied December 2006 formulary listings for general pharmacare programs in all but the smallest Canadian province. We characterized formularies in terms of the simple percentage of all available drugs that were listed on them and by a similar percentage that weighted each drug by its total national retail sales during 2006. We measured agreement among formularies using conventional inter-rater reliability scores (Kappa statistics) and a simple coverage-agreement measure. RESULTS: Provincial formularies studied here listed between 55% and 73% of the 796 drugs analyzed. When formulary listings were weighted by national retail sales, the measure of formulary coverage exceeded 86% in all provinces studied. Conventional inter-rater reliability scores (Kappa statistics) indicate that coverage agreement among most provincial formularies was low to moderate; however, drugs that were listed on all nine provincial formularies studied accounted for 77% of total retail spending in Canada. When analyzed by therapeutic category, the extent of coverage offered was relatively consistent across provinces in all but three leading categories: anti-migraine drugs, anti-dementia drugs and sedatives. CONCLUSION: While variations in coverage for specific drug classes and drug products remain important areas for investigation and policy consideration, Canada is currently operating with a significant "implicit national formulary" by way of the fact that provincial formularies independently yet mutually list most of the top-selling medicines in the marketplace.