How legalized marijuana is affecting our society has no clear answers, scientists and public health experts say — mainly because we don't have enough information yet.

In Colorado, state-sanctioned sales to any adult have been legal only since Jan. 1, 2014. Massachusetts, where voters approved a ballot initiative last year, won't see retail sales until July 2018.

Studies have shown both increases and decreases in youth and adult use, unreliable law-enforcement data about crashes and uncertainty about whether medical marijuana does what its backers claim.

While marijuana evangelists often deny that the drug could hurt anyone, some drug treatment experts say that when highly concentrated, it can cause psychosis.

Medical benefits

Marijuana appears to be an effective treatment for chronic pain, nausea and symptoms of multiple sclerosis, according to a January 2017 report from some of the nation’s top doctors and public health experts.

Commissioned by the National Academies of Sciences, Engineering and Medicine, the report said “conclusive or substantial” research backs the effectiveness of cannabis for those three conditions.

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But the report also warns of dangers: Increased risk of car crashes, lower birth weights and problems with memory and attention. It found strong connections between heavy cannabis use and the development of schizophrenia and other psychoses.

A variety of state and federal government agencies helped pay for the report, which included research from medical doctors, mental health practitioners and addiction specialists. Authors repeatedly noted that data on marijuana use is limited and scientists need more information as more and more states legalize medical marijuana.

"Conclusive evidence regarding the short- and long-term health effects (harms and benefits) of cannabis use remains elusive. A lack of scientific research has resulted in a lack of information on the health implications of cannabis use, which is a significant public health concern for vulnerable populations such as adolescents and pregnant women,” the report said.

Deborah Hasin, an epidemiology professor at Columbia University’s Mailman School of Public Health, has performed studies that found the prevalence of marijuana use disorders doubled between 2001 and 2013.

“If you increase the prevalence of users, you are going to increase the prevalence of people who have adverse consequences,” she said.

Youth use

But the data on which Hasin based her most recent study also showed slight drops in youth marijuana use in Colorado in the years following legalization. Voters legalized the drug in 2012, but sales didn't start until about 14 months later.

In Colorado, the percentage of teens 12 to 17 who had used marijuana in the previous month dropped from 12.6% in 2012-13 to 11.1% in 2014-15. In the same period, teens' past-year use dropped twice as fast, from 20.8% to 18.8%.

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In Washington, which legalized recreational marijuana in 2012, 17% of high school sophomores surveyed in 2016 reported having used marijuana in the previous month, down from 20% in 2010, according to the annual Washington State Healthy Youth Survey.

Nationally, 7.2% of teens reported using marijuana the previous month in 2014-15, according to the National Survey on Drug Use and Health from that year. The survey is the most recent one available from the federal Substance Abuse and Mental Health Services Administration.

Crime and public safety

Colorado Attorney General Cynthia Coffman said she believes data collection is lagging everywhere in the marijuana industry.

As the top law enforcement officer for the first state to legalize pot, Coffman worries about small towns struggling with the effects of major growing operations and marijuana users moving to big cities without jobs or housing.

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“Has the sky fallen? No it hasn’t, but there has been a cultural change," she said. "We’ve seen it in small towns and big cities. ...

"I would be pleased if it didn’t have a significant effect on crime, on DUI, on kids. It would make me happy to be wrong," she said. "I live in the world that sees the consequences.”

Dale Mondary, a police chief in Desert Hot Springs, Calif. — a former Drug Abuse Resistance Education officer now policing California's first city to allow commercial marijuana growing operations — remains deeply concerned about what will happen when people drive high.

First, he fears they will cause crashes. And second, because marijuana intoxication has no standard like blood-alcohol content for booze, he fears that his officers will be spending more and more time in courtrooms.

National Highway Traffic Safety Administration officials have said they believe drugs are increasingly a factor in fatal crashes though more research is needed. In 2015, about 1 in 5 of more than 31,000 fatal crashes in the U.S. involved at least one driver who tested positive for drugs — up from 12% in 2005.

A separate federal study of 11,000 weekend, nighttime drivers found 15.1% tested positive for illegal drugs in 2013 and 2014, up from 12.4% in 2007. Marijuana represented the largest increase: 12.6% tested positive in 2013 and 2014, up from 8.6% in 2007.

Mondary also is worried about security at dispensaries and the persistence of the black market.

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"I’m still very, very concerned about the recreational use. And frankly I’m opposed to the recreational use," he said.

But part of his job now is to protect the industry and its customers.

"If the city is going to allow it, I need to make sure we keep that product and our community just as absolutely safe as we can get it," he said.

Learning from other states

Mason Tvert, a spokesman for the pro-legalization Marijuana Policy Project who helped pass the Colorado law, believes concerns about marijuana are overblown and said the vast majority of users consume it responsibly.

The increasing acceptance of marijuana legalization reflects the reality that marijuana is far safer than many other widely accepted drugs, from prescription opiates like OxyContin to alcohol, he said.

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“There’s more use overall because people are recognizing that marijuana use is not as harmful as they were led to believe,” Tvert said. Many police officers have struggled to accept that voters have chosen to legalize marijuana, and many skeptics' claims have proven false.

And the war on drugs has brought innumerable negative consequences across the country, particularly for minority communities, he said.

State lawmakers are watching early adopters Colorado, Oregon and Washington, John Hudak of the Brookings Institution said. Some states have sought to preempt disconcerting trends, especially involving kids.

In the months after legalization, Colorado saw a jump in the number of children hospitalized for marijuana poisoning. The state later put new packaging and labeling regulations in place.

Now states such as Massachusetts, where lawmakers are crafting their own recreational pot rules, are writing those kinds of regulations into the laws before the first marijuana products get sold.

The debate is no longer about whether marijuana legalization will expand but what steps state legislators take to manage the risks that can accompany the industry, Hudak said.

"The march toward reform is an obvious one,” he said.

Contributing: Rosalie Murphy and Kristen Hwang, (Palm Springs, Calif.) Desert Sun; Nathan Bomey, USA TODAY. Follow Trevor Hughes on Twitter: @TrevorHughes

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