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.

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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. 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.

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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 struggling 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?

How do you tax it? What quality control standards do you set? How do you protect children while giving grown-ups the go-ahead to light up? What about driving under the influence? Can growers take business tax deductions? Who can grow pot, and how much? Where can you use it? Can cities opt out? Can workers be fired for smoking marijuana when they’re off duty? What about taking pot out of state? The list goes on.

The overarching question has big national implications. How do you do all of this without inviting the wrath of the federal government, which has been largely silent so far on how it will respond to a gaping conflict between U.S. and state law?

The Justice Department began reviewing the matter after last November’s election and repeatedly has promised to respond soon. But seven months later, states still are on their own, left to parse every passing comment from the department and President Obama.

In December, Obama said in an interview 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.”

In April, Attorney General Eric Holder said to Congress, “We are certainly going to enforce federal law. … When it comes to these marijuana initiatives, I think among the kinds of things we will have to consider is the impact on children.” He also mentioned violence related to drug trafficking and organized crime.

In May, Obama told reporters: “I honestly do not believe that legalizing drugs is the answer. But I do believe that a comprehensive approach – not just law enforcement, but prevention and education and treatment – that’s what we have to do.”

Rep. Jared Polis, a Colorado Democrat who favors legalization, predicts Washington will take a hands-off approach, based on Obama’s comments about setting law enforcement priorities.

“We would like to see that in writing,” Polis says. “But we believe, given the verbal assurances of the president, that we are moving forward in Colorado and Washington in implementing the will of the voters.”

The federal government 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 warned 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.

Federal agents in recent years have raided storefront dispensaries in California and Washington, seizing cash and pot. In April, the Justice Department targeted 63 dispensaries in Santa Ana, Calif., and filed three asset forfeiture lawsuits against properties housing seven pot shops. Prosecutors also sent letters to property owners and operators of 56 other marijuana dispensaries warning that they could face similar lawsuits.

University of Denver law professor Sam Kamin says if the administration doesn’t act soon to sort out the federal-state conflict, it may be too late to do much.

“At some point, it becomes so prevalent and so many citizens will be engaged in it that it’s hard to recriminalize something that’s become commonplace,” he says.

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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. In Colorado, for example, last November more people voted for legalized pot (55 percent) than voted for Obama (51 percent), which could help explain why the president was silent on marijuana before the election.

“We’re going to get a cultural divide here pretty quickly,” says Greg Strimple, a Republican pollster based in Boise, Idaho, who predicts Obama will duck the issue as long as possible.

Despite increasing public acceptance of marijuana, and growing interest in its potential therapeutic uses, politicians know there are complications that could come with commercializing an addictive substance, some of them already evident in medical marijuana states. Opponents of pot are particularly worried that legalization will result in increased adolescent use as young people’s estimations of the drug’s dangers decline.

“There’s no real win on this from a political perspective,” says Sabet. “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.

See Change Research, which analyzes the marijuana business, has estimated the national market for medical marijuana alone at $1.7 billion for 2011 and has projected it could reach $8.9 billion in five years. Overall, marijuana users spend tens of billions of dollars a year on pot, experts believe.

Ultimately, marijuana advocates say, it’s Congress that needs to budge, aligning federal laws with those of states moving to legalization. But that doesn’t appear likely anytime soon.

The administration appears uncertain how to proceed.

“The executive branch is in a pickle,” Rep. Ed Perlmutter, D-Colo., said at a recent news conference outside the Capitol with pot growers visiting town to lobby for changes. “Twenty-one states have a different view of the use of marijuana than the laws on the books for the federal government.”

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While the federal government hunkers down, Colorado and Washington state are moving forward on their own.

Colorado’s governor in May signed a set of bills to regulate legal use of the drug, and the state’s November ballot will ask voters to approve special sales and excise taxes on pot. In Washington state, the Liquor Control Board is drawing up rules covering everything from how plants will be grown to how many stores will be allowed. It expects to issue licenses for growers and processors in December, and impose 25 percent taxes three times over – when pot is grown, processed and sold to consumers.

“What we’re beginning to see is the unraveling of the criminal approach to marijuana policy,” says Tim Lynch, director of the libertarian Cato Institute’s Project on Criminal Justice. But, Lynch adds, “the next few years are going to be messy. There are going to be policy battles” as states work to bring a black market industry into the sunshine, and Washington wrestles with how to respond.

Already, a federal judge has struck down a Colorado requirement that pot magazines such as High Times be kept behind store counters, like pornography.

Marijuana advocates in Washington state, where officials have projected the legal pot market could bring the state a half-billion a year in revenue, are complaining that state regulators are still banning sales of hash or hash oil, a marijuana extract.

Pot growers in medical marijuana states are chafing at federal laws that deny them access to the banking system, tax deductions and other opportunities that other businesses take for granted. Many dispensaries are forced to operate on a cash-only basis, which can be an invitation to organized crime.

It’s already legal for adults in Colorado and Washington to light up at will, as long as they do so in private.

That creates all kinds of new challenges for law enforcement.

Pat Slack, a commander with the Snohomish County Regional Drug Taskforce in Washington state, said local police are receiving calls about smokers flouting regulations against lighting up in public. In at least one instance, Slack said, that included a complaint about a smoker whose haze was wafting over a backyard fence and into the middle of a child’s birthday party. But with many other problems confronting local officers, scofflaws are largely being ignored.

“There’s not much we can do to help,” Slack says. “A lot of people have to get accustomed to what the change is.”

In Colorado, Tom Gorman, director of the federal Rocky Mountain High Intensity Drug Taskforce, takes a tougher stance on his state’s decision to legalize pot.

“This is against the law, I don’t care what Colorado says,” Gorman said. “It puts us in a position, where you book a guy or gal and they have marijuana, do you give it back? Do you destroy it? What in effect I am doing by giving it back is I am committing a felony. If the court orders me to return it, the court is giving me an illegal order.”

More than 30 pot growers and distributors, going all-out to present a buttoned-down image in suits and sensible pumps rather than ponytails and weed T-shirts, spent two days on Capitol Hill in June lobbying for equal treatment under tax and banking laws and seeking an end to federal property seizures.

“It’s truly unfortunate that the Justice Department can’t find a way to respect the will of the people,” says Sean Luse of the 13-year-old Berkeley Patients Group in California, a multimillion-dollar pot collective whose landlord is facing the threat of property forfeiture.

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As Colorado and Washington state press on, 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. Regulation has been lax, leading some overwhelmed communities to complain about too-easy access from illegal storefront pot shops and related problems such as loitering and unsavory characters. That prompted cities around the state to say enough already and ban dispensaries. Pot advocates sued.

In May, the California Supreme Court ruled unanimously 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. By contrast, whitepages.com lists 112 Starbucks in the city.

This isn’t full-scale buyer’s remorse, but more a course correction before the inevitable next push to full-on legalization in the state.

Baker Montgomery, a member of the Eagle Rock neighborhood council in Los Angeles, where pot shops were prevalent, said May’s vote to limit the number of shops was all about ridding the city of illicit dispensaries.

“They’re just not following what small amounts of rules there are on the books,” Montgomery said.

In 2010, California voters opted against legalizing marijuana for recreational use, drawing the line at medical use.

But Jeffrey Dunn, a Southern California attorney who represented cities in the Supreme Court case, says that in reality the state’s dispensaries have been operating so loosely that already “it’s really all-access.”

At the Venice Beach Care Center, one of the dispensaries that will be allowed to stay open in Los Angeles, founding director Brennan Thicke believes there still is widespread support for medical marijuana in California. But he says the state isn’t ready for more just yet.

“We have to get (medical) right first,” Thicke said.

Dunn doubts that’s possible.

“What we’ve learned is, it is very difficult if not impossible to regulate these facilities,” he said.

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Other states, Colorado among them, have had their own bumps in the road with medical marijuana.

A Denver-area hospital, for example, saw children getting sick after eating treats and other foods made with marijuana in the two years after a 2009 federal policy change led to a surge in medical marijuana use, according to a study in JAMA Pediatrics in May. In the preceding four years, the hospital had no such cases.

The Colorado Education Department reported a sharp rise in drug-related suspensions and expulsions after medical marijuana took off. An audit of the state’s medical marijuana system found the state had failed to adequately track the growth and distribution of pot or to fully check out the backgrounds of pot dealers.

“What we’re doing is not working,” says Dr. Christian Thurstone, a psychiatrist whose Denver youth substance abuse treatment center has seen referrals for marijuana double since September. In addition, he sees young people becoming increasingly reluctant to be treated, arguing that it can’t be bad for them if it’s legal.

Yet Daniel Rees, a researcher at the University of Colorado Denver, analyzed data from 16 states that have approved medical marijuana and found no evidence that legalization had increased pot use among high school students.

In looking at young people, Rees concludes: “Should we be worried that marijuana use nationally is going up? Yes. Is legalization of medical marijuana the culprit? No.”

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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.

Safer-than-alcohol was “the message that won the day” with voters in Colorado, says Tvert.

For others, money talks: Why let drug cartels rake in untaxed profits when a cut of that money 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; some reflect back on what they see as their own harmless experience in their youth. They worry about high school kids getting an arrest record that will haunt them for life. They see racial inequity in the way marijuana laws are enforced. They’re weary of the “war on drugs,” and want law enforcement to focus on other areas.

“I don’t plan to use marijuana, but it just seemed we waste a lot of time and energy trying to enforce something when there are other things we should be focused on,” says Sherri Georges, who works at a Colorado Springs, Colo., saddle shop. “I think that alcohol is a way bigger problem than marijuana, especially for kids.”

Opponents have retorts at the ready.

They point to a 2012 study finding that regular use of marijuana during teen years can lead to a long-term drop in IQ, and a different study indicating marijuana use can induce and exacerbate psychotic illness in susceptible people. They question the idea that regulating pot will bring in big money, saying revenue estimates are grossly exaggerated.

They counter 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. Slack said the vast majority of people jailed for marijuana possession were originally charged with dealing drugs and accepted plea bargains for possession. The average possession charge for those in jail is 115 pounds, Slack says, which he calls enough for “personal use for a small city.”

Over and over, marijuana opponents warn that baby boomers who are drawing on their own innocuous experiences with pot are overlooking the much higher potency of the marijuana now in circulation.

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. An estimated 9 percent of people who try marijuana eventually become addicted, and the numbers are higher for those who start using pot when they are young. That’s less than the addiction rates for nicotine or alcohol, but still significant.

“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. “But it’s not about that. 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.”

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So how bad, or good, is pot?

There are studies that set off medical alarm bells but also studies that support the safer-than-alcohol crowd and suggest promising therapeutic uses.

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. In a 22-page article for Mayo Clinic Proceedings in 2012, he laid out the contradictions in U.S. policy and declared that “little about cannabis is straightforward.”

“Anybody can find data to support almost any position,” Bostwick says now.

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.

Marijuana is a Schedule I drug under 1970 law, meaning the government deems it to have “no currently accepted medical use” and a “high potential for abuse.” The only federally authorized source of marijuana for research is grown at the University of Mississippi, and the government tightly regulates its use. The National Institute on Drug Abuse says plenty of work with cannabis is ongoing, but Bostwick says federal restrictions have caused a “near-cessation of scientific research.”

The American Medical Association opposes legalizing pot, calling it a “dangerous drug” and a public health concern. But it also is urging the government to review marijuana’s status as a Schedule 1 drug in the interest of promoting more research.

“The evidence is pretty clear that in 1970 the decision to make the drug illegal, or put it on Schedule I, was a political decision,” says Bostwick. “And it seems pretty obvious in 2013 that states, making their decisions the way they are, are making political decisions. Science is not present in either situation to the degree that it needs to be.”

The National Institute on Drug Abuse’s director, Dr. Nora Volkow, says that for all the potential dangers of marijuana, “cannabinoids are just amazing compounds, and understanding how to use them properly could be actually very beneficial therapeutically.” But she worries that legalizing pot will result in increased use of marijuana by young people, and impair their brain development.

“You cannot mess around with the cognitive capacity of your young people because you are going to rely on them,” she says. “Think about it: Do you want a nation where your young people are stoned?”

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As state after state moves toward a more liberal approach to marijuana, the turnaround is drawing comparisons to shifting attitudes on gay marriage, for which polls find rapidly growing acceptance, especially among younger voters. That could point toward durable majority support as this population ages. Gay marriage is now legal in 12 states and Washington, D.C.

On marijuana, “we’re having a hard time almost believing how fast public opinion is changing in our direction,” says Nadelmann of the Drug Policy Alliance.

But William Galston and E.J. Dionne, who co-wrote a paper on the new politics of marijuana for the Brookings Institution, believe marijuana legalization hasn’t achieved a deep enough level of support to suggest a tipping point, with attitudes toward legalization marked by ambivalence and uncertainty.

“Compared with attitudes toward same-sex marriage, support for marijuana legalization is much less driven by moral conviction and much more by the belief that it is not a moral issue at all,” they wrote.

No one expects Congress to change federal law anytime soon.

Partisans on both sides think people in other states will keep a close eye on the precedent-setting experiment underway in Colorado and Washington as they decide whether to give the green light to marijuana elsewhere.

“It will happen very suddenly,” predicts the Cato Institute’s Lynch. “In 10-15 years, it will be hard to find a politician who will say they were ever against legalization.”

Sabet worries that things will move so fast that the negative effects of legalization won’t yet be fully apparent when other states start giving the go-ahead to pot. He’s hoping for a different outcome.

“I actually think that this is going to wake a lot of people up who might have looked the other way during the medical marijuana debate,” he says. “In many ways, it actually might be the catalyst to turn things around.”

Past predictions on pot have been wildly off-base, in both directions.

The 1972 commission that recommended decriminalizing marijuana speculated pot might be nothing more than a fad.

Then there’s “Reefer Madness,” the 1936 propaganda movie that pot fans rediscovered and turned into a cult classic in the 1970s. It labeled pot “The Real Public Enemy Number One!”

The movie spins a tale of dire consequences “leading finally to acts of shocking violence … ending often in incurable insanity.”

Associated Press writers Kristen Wyatt in Denver, Gene Johnson in Seattle, Lauran Neergaard in Washington and AP researcher Monika Mathur in Washington contributed to this report.