Thomson Reuters Pushes for states to legalize marijuana have been growing in popularity. After a legalization referendum came into effect on January 1, 2018, California became the largest regulated cannabis market in the world. That could translate into a "green rush" of new investment into the marijuana industry in California. Across the United States, there’s a seemingly unstoppable movement gaining ground. The legalization of cannabis for adult use – first passed in the states of Washington and Colorado in 2012 – is now approved in a total of nine states and the District of Columbia.