California is set to ring in the new year with its first legal recreational marijuana sales.
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With a population of 39.5 million, California's new ruling means more than one in five Americans now live in states where those over 21 can purchase marijuana.
California is the sixth US state to allow the sale of cannabis products beyond medicinal purposes.
Colorado, Washington, Oregon, Alaska, and Nevada previously legalised recreational weed sales, with Massachusetts and Maine set to follow later in the year.
The rule change comes more than two decades after California passed the country's first medical marijuana law.
The state's marijuana market has been valued at several billion dollars and is set to bring in around US$1 billion (NZ$1.4 billion) annual tax revenue.
However, the drug still remains classified as an illegal narcotic under US federal law.
According to a 2016 report by the American Civil Liberties Union, the year's marijuana arrests outnumbered those for violent crime.
Many California cities including Los Angeles and San Francisco won't be ready to sell just yet, as retailers still have to apply for their state licenses.
But that hasn't stopped enthusiasts from gathering to celebrate 'Green Monday'.
California resident Michael Sherman told CBS News "it's exciting".
"To be able to have that privilege to not have to go through all the ins and outs to be able to achieve what I need and I want in my life, it's amazing.
"I'll pay the taxes, give it to the government, whatever they want to spend it on, as long as I get what I need."
Another California resident, Aatar De Sa, told CBS News it's a "historic day".
"We're the last generation to grow up with weed illegally, we got in trouble as teenagers, it's the end of a prohibition...
"It's an acceptance of something that's been a large part of the culture."