The aim of this thesis is to estimate the impact of the External Reference Pricing (ERP) implemented in the Colombian pharmaceutical industry on the prices and quantities sold of the regulated products and their unregulated closest substitutes using longitudinal data for the period 2011-2018. To achieve this, I investigate the policy designed by the Health Ministry in September 2013 to regulate a set of pharmaceuticals when they were of national interest or considered to be a threat to the financial sustainability of the system. The methodology has been implemented four times since it was established, comprising over 1,000 different commercial presentations. According to the ministry, the main intention of this policy was to contain the recent growth of the pharmaceutical expenditure and to contribute to a better access to drugs.