California legalised cannabis for medical use in 1996, but is now permitting recreational use by adults.
California has become the largest state in the US to legalise recreational cannabis use.
As of 1 January 2018, adults aged over 21 can possess up to an ounce (28 grams) of the drug and can grow up to six marijuana plants at home.
Opponents say the law will lead to more driving under the influence of the drug and introduce young people to drug use.
But business is eyeing what could be an industry worth tens of billions of dollars in the next few years.
'First years will be a mess'
Californians voted in favour of Proposition 64 legalising cannabis 14 months ago, in a poll that took place alongside the US presidential vote.
Since then, a complicated patchwork of taxes and regulations has been drawn up to govern sales of the drug.
Critics say the red tape will discourage consumers, growers and retailers from leaving the state's vast black market and only a few dozen shops have so far been approved to open.
The cities of Los Angeles and San Francisco have yet to sanction any recreational marijuana outlets.
"The first year, two years are going to be a mess," as cities agree their own regulations and supply and demand is established, Troy Dayton told AFP news agency.
He is the CEO of Arcview, a company that analyses the global cannabis market. He predicted there could be "crazy fluctuation in price over the first year or two".
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Nonetheless, the potential rewards are huge in the US's wealthiest and most populous state.
In 1996, California was the first state to legalise marijuana for medicinal use.
The market for medical marijuana products has already led to a surge in the range of available products
But Arcview estimates the illegal cannabis market is currently worth an annual $5.1bn (£3.8bn) and once legalised could be worth $5.8bn by 2021.
Legal buyers in the newly created "adult use" market will also now have to pay state, sales and municipal taxes which it is estimated will generate $1bn per year.
They will be prohibited from consuming the drug in public places, with 1,000 feet (300m) of a school or while driving.
With six US states - Colorado, Washington, Oregon, Alaska and Nevada as well as California - having legalised cannabis, one in five Americans can now legally purchase the drug.
Despite this, the federal government still regards marijuana as illegal, classifying it alongside heroin and cocaine as a proscribed substance.
Originally published here: http://www.bbc.com/news/world-us-canada-42532776