The Mexican institute for social security IMSS represents alone 44% of the total health care market in the country. Selling to this major client requires each drug to be listed in the national drug formulary according to a law passed in 2002. The objective of this law was to certify not only the safety profile of every new drug, but also its economical benefits based on a pharmacoeconomical study that must be presented for evaluation. OBJECTIVES: To analyze if the expenditure made by the IMSS in 2011 used the cost-effectiveness criteria based on the drugs listed in the national formulary. METHODS: Six high resource consuming diseases in the country were selected together with their respective prescribed treatments. Each prescribed treatment was classified according to three parameters: - Less expensive treatment - Most expensive treatment - Cost-effective treatment Prices of each treatment and its total expenditure were obtained through the institute’s official purchase portal. Pharmacoeconomical studies needed to obtain the cost effective treatment were already evaluated for drug's inclusion in the national formulary.  Results were analyzed to evaluate the expenditure's nature. RESULTS: In a half of the studied diseases, the expenditure on other none cost-effective treatments represented at least 50% more than for the cost-effective ones. The total expenditure on the studied cases was US $226,881,190.64. From these consumed resources, 33.54% was used to buy other non cost-effective drugs. From the observed purchases made, 40% were direct adjudications not open to competition. CONCLUSIONS: Even though every new drug is being evaluated since year 2002 to show its economical benefits, the studied institution in not fully using the cost-effectiveness criteria to buy its drugs in Mexico, even in some of the most resource consuming diseases.