It will now be legal to buy recreational marijuana in the state of California.
The law, which came into effect this morning, makes California the sixth US territory where the consumption and trade of marijuana is legal.
The US Treasurer's Office estimates that in 2020, legal marijuana sales in California will reach $US7.2 billion ($A10.1 billion) and will generate some $US1 billion ($A1.4 billion) in taxes.
In Nevada, sales of legal marijuana generated more than $US19 million ($A27 million) in taxes in the first four months.
The dispensaries that have already received the green light to operate from January 1 are located mainly in northern California, many in the San Francisco Bay area, as well as in the southern counties of Riverside and San Diego.
In addition to California, the sale of marijuana for recreational use is legal in the states of Alaska, Colorado, Nevada, Oregon and Washington, while it is also legal in Maine to have a personal use amount, with retailing expected to begin in mid-2018, while in Massachusetts it will be legal as of July.