WASHINGTON -- It took 50 years for American attitudes about marijuana to zigzag from the paranoia of "Reefer Madness" to the excesses of Woodstock back to the hard line of "Just Say No."
The next 25 years took the nation from Bill Clinton, who famously "didn't inhale," to Barack Obama, who most emphatically did.
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Now, in just a few short years, public opinion has moved so dramatically toward general acceptance that even those who champion legalization are surprised at how quickly attitudes are changing and states are moving to approve the drug -- for medical use and just for fun.
It is a moment in America that is rife with contradictions:
• People are looking more kindly on marijuana even as science reveals more about the drug's potential dangers, particularly for young people.
• States are giving the green light to the drug in direct defiance of a federal prohibition on its use.
• Exploration of the potential medical benefit is limited by high federal hurdles to research.
Washington policymakers seem reluctant to deal with any of it.
Richard Bonnie, a University of Virginia law professor who worked for a national commission that recommended decriminalizing marijuana in 1972, sees the public taking a big leap from prohibition to a more laissez-faire approach without full deliberation.
"It's a remarkable story historically," he says. "But as a matter of public policy, it's a little worrisome."
More than a little worrisome to those in the anti-drug movement.
"We're on this hundred-mile-an-hour freight train to legalizing a third addictive substance," says Kevin Sabet, a former drug policy adviser in the Obama administration, lumping marijuana with tobacco and alcohol.
Legalization strategist Ethan Nadelmann, executive director of the Drug Policy Alliance, likes the direction the marijuana smoke is wafting. But knows his side has considerable work yet to do.
"I'm constantly reminding my allies that marijuana is not going to legalize itself," he says.
By the numbers:
Eighteen states and the District of Columbia have legalized the use of marijuana for medical purposes since California voters made the first move in 1996. Voters in Colorado and Washington state took the next step last year and approved pot for recreational use. Alaska is likely to vote on the same question in 2014, and a few other states are expected to put recreational use on the ballot in 2016.
Nearly half of adults have tried marijuana, 12 percent of them in the past year, according to a survey by the Pew Research Center.
Fifty-two percent of adults favor legalizing marijuana, up 11 percentage points just since 2010, according to Pew.
Sixty percent think Washington shouldn't enforce federal laws against marijuana in states that have approved its use.
Where California led the charge on medical marijuana, the next chapter in this story is being written in Colorado and Washington state.
Policymakers there are grappling with all sorts of sticky issues revolving around one central question: How do you legally regulate the production, distribution, sale and use of marijuana for recreational purposes when federal law bans all of the above?
The Justice Department began reviewing the matter after last November's election. But seven months later, states still are on their own.
Both sides in the debate paid close attention when Obama said in December that "it does not make sense, from a prioritization point of view, for us to focus on recreational drug users in a state that has already said that under state law that's legal."
Rep. Jared Polis, a Colorado Democrat who favors legalization, predicts Washington will take a hands-off approach, based on Obama's comments. But he's quick to add: "We would like to see that in writing."
The federal government already has taken a similar approach toward users in states that have approved marijuana for medical use.
It doesn't go after pot-smoking cancer patients or grandmas with glaucoma. But it also has made clear that people who are in the business of growing, selling and distributing marijuana on a large scale are subject to potential prosecution for violations of the Controlled Substances Act -- even in states that have legalized medical use.
There's a political calculus for the president, or any other politician, in all of this.
Younger people, who tend to vote more Democratic, are more supportive of legalizing marijuana, as are people in the West, where the libertarian streak runs strong.
Despite increasing public acceptance of marijuana overall, politicians know there are complications that could come with commercializing an addictive substance. Opponents of pot are particularly worried that legalization will result in increased use by young people.
Sabet frames the conundrum for Obama: "Do you want to be the president that stops a popular cause, especially a cause that's popular within your own party? Or do you want to be the president that enables youth drug use that will have ramifications down the road?"
Marijuana legalization advocates offer politicians a rosier scenario, in which legitimate pot businesses eager to keep their operating licenses make sure not to sell to minors.
"Having a regulated system is the only way to ensure that we're not ceding control of this popular substance to the criminal market and to black marketeers," says Aaron Smith, executive director of the National Cannabis Industry Association, a trade group for legal pot businesses in the U.S.
States forge ahead
While the federal government hunkers down, Colorado and Washington state are moving forward on their own with regulations covering everything from how plants will be grown to how many stores will be allowed.
Tim Lynch, director of the libertarian Cato Institute's Project on Criminal Justice, predicts "the next few years are going to be messy" as states work to bring a black-market industry into the sunshine.
California's experience with medical marijuana offers a window into potential pitfalls that can come with wider availability of pot.
Dispensaries for medical marijuana have proliferated in the state, and regulation has been lax, prompting a number of cities around the state to ban dispensaries.
In May, the California Supreme Court ruled that cities and counties can ban medical marijuana dispensaries. A few weeks later, Los Angeles voters approved a ballot measure that limits the number of pot shops in the city to 135, down from an estimated high of about 1,000.
This isn't full-scale buyer's remorse, but more a course correction before the inevitable next push for full-on legalization in the state.
Supporters, not users
Growing support for legalization doesn't mean everybody wants to light up: Barely one in 10 Americans used pot in the past year.
Those who do want to see marijuana legalized range from libertarians who oppose much government intervention to people who want to see an activist government aggressively regulate marijuana production and sales.
For some, money talks: Why let drug cartels rake in untaxed profits when a cut could go into government coffers?
There are other threads in the growing acceptance of pot.
People think it's not as dangerous as once believed. They worry about high school kids getting an arrest record. They see racial inequity in the way marijuana laws are enforced. They're weary of the "war on drugs."
Opponents counter with a 2012 study finding that regular use of marijuana during teen years can lead to a long-term drop in IQ, and another study indicating marijuana use can induce and exacerbate psychotic illness in susceptible people. They question the notion that regulating pot will bring in big money, saying revenue estimates are grossly exaggerated.
They reject the claim that prisons are bulging with people convicted of simple possession by citing federal statistics showing only a small percentage of federal and state inmates are behind bars for that alone.
They warn that baby boomers who draw on their own innocuous experiences with pot are overlooking the much higher potency of today's marijuana.
In 2009, concentrations of THC, the psychoactive ingredient in pot, averaged close to 10 percent in marijuana, compared with about 4 percent in the 1980s, according to the National Institute on Drug Abuse.
"If marijuana legalization was about my old buddies at Berkeley smoking in People's Park once a week I don't think many of us would care that much," says Sabet, who helped to found Smart Approaches to Marijuana, a group that opposes legalization. "It's really about creating a new industry that's going to target kids and target minorities and our vulnerable populations just like our legal industries do today."
So how bad, or good, is pot?
J. Michael Bostwick, a psychiatrist at the Mayo Clinic, set out to sort through more than 100 sometimes conflicting studies after his teenage son became addicted to pot, and turned his findings into a 22-page article for Mayo Clinic Proceedings in 2012.
For all of the talk that smoking pot is no big deal, Bostwick says he determined that "it was a very big deal. There were addiction issues. There were psychosis issues.
But there was also this very large body of literature suggesting that it could potentially have very valuable pharmaceutical applications but the research was stymied" by federal barriers.
The National Institute on Drug Abuse says research is ongoing.
Dr. Nora Volkow, the institute's director, worries that legalizing pot will result in increased use of marijuana by young people, and impair their brain development.
"Think about it: Do you want a nation where your young people are stoned?" she asks.
Partisans on both sides think people in other states will keep a close eye on Colorado and Washington as they decide what happens next.
But past predictions on pot have been wildly off-base.
"Reefer Madness," the 1936 propaganda movie that pot fans turned into a cult classic in the 1970s, spins a tale of dire consequences "ending often in incurable insanity."
Marijuana: Washington, Colorado are test cases for the country