Legalized marijuana The nation's capital, Washington DC, is one of the more than 20 states that have legalized marijuana. If you want more information on that, read on. - The "Legalization of Marijuana for Medical Treatment Amendment Act of 2010" or Amendment Act B18-622 was approved on May 4, 2010, after receiving a 13-0 vote from the Council of the District of Columbia. It was then signed by the Mayor on May 21, 2010, and made effective on July 27, 2010.  - People can register ...