This article analyzes the circulation of substances like cocaine, morphine, opium, and marijuana (cannabis sativa) in Chile between 1920 and 1960. It underscores the development of circuits and organizations involved in the production, importation, and traffic of legally and medically-regulated substances as well as the implementation of laws and institutions charged with regulating them. In this way, the article analyzes the role played by doctors and pharmacists in the constitution of an illegal market for drugs and the emergence of specialized traffickers and police organizations dedicated to persecuting them. All this occurred in the context of a legal system that tolerated the regulated prescription of these substances and that, at the same time, witnessed the continual violation of these norms.