This work results from careful research on scientific conceptions, political reasons and social mechanisms that contributed to the prohibition of marijuana in Brazil in the twentieth century, The author traces controversy, evidencing how the progressive criminalization around the uses of the herb created land for the association of the plant with stigmas attributed to the black population in the post-abolition. Throughout medical theses, the author also makes us perceive the concerns, challenges and eugenics plans present in the process of criminalization of cannabis, revealing the political senses of scientific conclusions.