THE first recreational marijuana industry in the US has opened in Colorado, kicking off an experiment that will be watched closely around the world.
Activists hope to prove that legalisation is a better alternative than the costly American-led war on drugs.
"Honestly, I thought I'd never see the day," said a giddy Errin Reaume, who shared hits of concentrated marijuana at a 1920s-themed "Prohibition Is Over" party in Denver.
Sceptics worry the industry will make the drug more widely available to teens, even though legal sales are limited to adults over 21.
Colorado voters in 2012 approved the legal pot industry. Washington state has its own version, which is scheduled to open in mid-2014.
Pot advocates have argued a legal market would generate revenue for states and save money by not having to lock up so many drug offenders.
Colorado set up an elaborate plant-tracking system to try to keep the drug away from the black market, and regulators set up packaging, labelling and testing requirements, along with potency limits for edible pot.
Marijuana is still illegal under federal law, but the US Justice Department outlined an eight-point slate of priorities for pot regulation, requiring states to keep the drug away from minors, criminal cartels, federal property and other states in order to avoid a federal crackdown.
"We understand that Colorado is under a microscope," Jack Finlaw, lawyer to Governor John Hickenlooper, and overseer of a major task force to chart new pot laws, recently told reporters.
A group of addiction counsellors and physicians said they were seeing more marijuana addiction problems, especially in youths.
They also said that wider availability of the drug would exacerbate the problem.