Just two decades after medicinal use was legalised in California in 1996, the state has relaxed rules prohibiting its sale and ownership.
Known as the Adult Use of Marijuana Act (AUMA) or Proposition 64, the law change was voted for by Californians in 2016 despite a federal prohibition on the drug still being in effect in the US.
California is the latest state after Alaska, Colorado, Oregon, Nevada and Washington to legalise cannabis.
Adults aged 21 and over are allowed to grow up to six plants and possess up to an ounce of the drug.
Analysis by Californian authorities have estimated that it could increase tax revenue by up to $1bn (£740m), while reducing local government expenditures by tens of millions of dollars.
Opponents of the legalisation have warned that it risks introducing young people to drug use and may lead to a rise in drug driving offences.
“This is something we’ve all been waiting for,” said Johnny Hernandez, a tattoo artist from Modesto.
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“It is something that can help so many people and there’s no reason why we should not be sharing that.”
The legalisation is not expected to seriously affect the black market in cannabis with taxes and fees on retail marijuana amounting to a price increase of as much as 70% for consumers.