Colorado voters legalized recreational pot use on Tuesday, setting up a clash with federal drug policy. A similar measure was also approved in Washington state, though voters in Oregon rejected recreational pot.
When state and federal laws conflict, federal law takes precedence. Federal authorities could sue in an attempt to block the measures in Colorado and Washington from taking effect.
It remained unclear how the federal government would respond.
"Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or Goldfish too quickly," Colorado Gov. John Hickenlooper, a Democrat who opposed the measure, said in a statement.
Marijuana backers weren't heeding the caution. A jubilant crowd erupted in cheers in a Denver bar almost as soon as polls closed.
"It means I'm going to smoke a lot of weed tonight, woo!" said marijuana amendment backer Gary Rymer.
Colorado's direct challenge to federal drug law comes 12 years after the state legalized marijuana for people with certain medical conditions.
The state's latest pot measure stated that adults over 21 could possess up to 1 ounce of marijuana, or six marijuana plants, for personal use.
The amendment also allows commercial sales of the drug starting next year, though cities would be free to prohibit commercial pot businesses. It specifically prohibits using the drug "openly or publicly."
Legalization backers argued that marijuana is no more dangerous than alcohol and should be regulated and taxed. Opponents said legalizing pot would increase its use and make it easier for youths to obtain it.
Colorado exit polls showed a generational divide on marijuana. Legalization was solidly supported by thirty-somethings but opposed by about two-thirds of those older than 65.
"I think it's a license for the younger generation to do drugs," said 67-year-old Republican Mary Ann Calisto, of Bennett, east of Denver.
But younger voters were swayed by arguments that marijuana could produce needed tax money for schools.
Stacie Packard, a 42-year-old mother of two from the Denver suburb of Wheat Ridge, said she preaches to her children to stay away from drugs. Ultimately, she decided to support it because the first $40 million raised in taxes on pot would go to public schools.
"I guess it's kind of a shame that it's come to this," she said about the education funding.
The amendment sets up an elaborate regulatory scheme for how the drug could be used and sold. The amendment directed state lawmakers to regulate the drug through the Department of Revenue, which already oversaw alcohol sales and medical marijuana dispensaries.
The measure also directed state lawmakers to place an excise tax of up to 15 percent on marijuana sales, with the first $40 million each year devoted to school construction.
After approval by the Legislature, the pot tax would face final approval by voters.
The amendment also allows the production of industrial hemp, a far less potent cousin of marijuana that can be used for fiber and fuel, among other things.
Nine states — Hawaii, Kentucky, Maine, Maryland, Montana, North Dakota, Oregon, Vermont and West Virginia — have already passed laws allowing hemp cultivation or research.
But federal drug law prohibits hemp production, so industrial hemp used in products from granolas to soaps is currently imported.
Colorado's amendment did not affect existing medical marijuana law.
Colorado has 536 dispensaries licensed statewide. The state had several dozen more before a federal crackdown began earlier this year, targeting dispensaries near schools.
Colorado voters rejected marijuana legalization in 2006, 59 percent to 41 percent.