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.
And 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. It's intriguing, it's interesting, it's good that liberalization is occurring, but it is 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 he 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.
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. More teenagers now say they smoke marijuana than ordinary cigarettes.
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. Seventy-two percent think government efforts to enforce marijuana laws cost more than they're worth.
"By Election Day 2016, we expect to see at least seven states where marijuana is legal and being regulated like alcohol," says Mason Tvert, a spokesman for the Marijuana Policy Project, a national legalization group.