Cannabis (marijuana) is listed in Schedule I of the 1970 Controlled Substance Act. Schedule 1 classification is supposed to mean: "(A) The drug or other substance has a high potential for abuse. (B) The drug or other substance has no currently accepted medical use in treatment in the United States. (C) There is a lack of accepted safety for use of the drug or other substance under medical supervision."
Source
U.S. Code. Title 21, Chapter 13 -- Drug Abuse Prevention and Control -- Section 812, Schedules of Controlled Substances, p. 384.
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