Marijuana will become legal in Colorado and Washington state for all uses.
Voters approved ballot measures to legalize and regulate the production, possession and distribution of marijuana for people aged 21 and older.
"Today the state of Washington looked at 70 years of marijuana prohibition and said it's time for a new approach," said Alison Holcomb, manager of the campaign that won passage of Initiative 502 in Washington.
Colorado Gov. John Hickenlooper, a Democrat who opposed legalization, was less enthused. "Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or gold fish too quickly," he said.
A similar measure was voted down in Oregon. Colorado and Washington are among the states that have allowed marijuana use for medical reasons.
Massachusetts will join those states after its electorate approved a ballot measure to allow medical marijuana use.
The marijuana measures in Colorado and Washington will likely pose a headache for the U.S. Justice Department and the Drug Enforcement Administration, which consider pot an illegal drug. The DOJ has declined to say how it would respond if the measures were approved.
Colorado's Amendment 64 will allow adults over 21 to possess up to an ounce of marijuana, though using the drug publicly would be banned. The amendment would allow people to grow up to six marijuana plants in a private, secure area.
Washington's measure establishes a system of state-licensed marijuana growers, processors and stores, where adults can buy up to an ounce. It also establishes a standard blood test limit for driving under the influence.
The Washington measure was notable for its sponsors and supporters, who ranged from public health experts and wealthy high-tech executives to two former top Justice Department's officials in Seattle, U.S. Attorneys John McKay and Kate Pflaumer.
"Marijuana policy reform remains an issue where the people lead and the politicians follow," said Ethan Nadelmann of the Drug Policy Alliance, which opposes the co-called "war on drugs." "But Washington state shows that many politicians are beginning to catch up."
Estimates show pot taxes could bring in hundreds of millions of dollars a year, but the sales won't start until state officials make rules to govern the legal weed industry.
The Washington measure was opposed by Derek Franklin, president of the Washington Association for Substance Abuse and Violence Prevention.
"Legalizing is going to increase marijuana use among kids and really create a mess with the federal government," Franklin said. "It's a bit of a tragedy for the state."
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