Two states recently legalized the sale of recreational marijuana, and experts are weighing in on the health effects of this policy shift.

On Jan. 1, Colorado became the first state in the U.S. to allow people to legally purchase marijuana for recreational use, with Washington state to do the same later this year. Currently, 20 states and the District of Columbia allow the use of marijuana for medical purposes, making it possible that other states will follow Colorado and Washington toward greater legalization of the drug.

With so many factors involved, it’s nearly impossible to predict the effects of this groundbreaking change on public health and safety, but the legalization of marijuana will likely have both positive and negative consequences.

The impact of marijuana legalization will depend, in large part, on how many people start using the drug once it becomes legal. The shift from illegal sales to a market-based system could open the flood gates to new users.

“It is very likely that the commercial price, even with the taxes that Colorado and Washington are imposing, will end up being substantially lower than the prices in the illegal market because production costs will be very much down, and distribution costs will be dramatically lower,” says Peter Reuter, Ph.D., a professor in the School of Public Policy and the Department of Criminology at the University of Maryland.

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In 2012, more than 30 percent of Americans between the ages of 18 and 25 reported using marijuana in the past year, according to the National Institute on Drug Abuse. For 12- to 17-year-olds, usage was nearly fourteen percent.

Reuter expects that legalization will only increase these numbers. “I’m certainly confident that there will be an increase in marijuana use and in heavy marijuana use,” he says.

It is also doubtful that teenagers will be deterred by the Colorado and Washington laws that prohibit the sale of marijuana to minors, when some have been using both marijuana and cigarettes illegally for many years.

Marijuana has several well-studied—if poorly presented—effects on public health and safety.

“The public is largely misinformed about the health and behavioral effects of marijuana,” says Roger Roffman, D.S.W., a professor emeritus in the School of Social Work at the University of Washington and author of the forthcoming book Marijuana Nation. “There are an awful lot of people who simply don’t know what science knows about marijuana.”

Key among these is that marijuana is addictive. According to the National Institute for Drug Abuse, estimates suggest that one in nine users become addicted to marijuana. For people who start using as teenagers, this increases to one in six.

“While marijuana dependence is not life-threatening,” says Roffman, “it can result in very serious adverse consequences for an individual in terms of their functioning in school, at work, in relationships. In addition, the compulsive use of marijuana—very heavy use—can have an adverse effect on cognitive functioning.”

Teenagers, in particular, are susceptible to the negative effects of marijuana because parts of their brain—such as the prefrontal cortex—are still developing. People who start using marijuana as teens are “potentially at risk of losing IQ points because of an interference in normal brain development that occurs in the teenage years,” says Roffman.

Another potential effect of marijuana legalization is an increase in the number of people driving while high. While marijuana use by drivers is not as dangerous as alcohol use, pot and cars still don’t mix.

“Being high and driving increases the risk of accidents,” says Roffman, “in part because of the effects on the driver’s motor performance, perception, and concentration.”

Other health effects of marijuana include an increased risk of heart attack and stroke for adults with cardiovascular disease and acute psychotic episodes in people with a history of schizophrenia.

As for the long-term effects of marijuana use, major studies have so far failed to address that question.

“We don’t know what the long-term effects of marijuana use are,” says Reuter, “and there’s clearly going to be an increase in the number of people who use marijuana on a regular basis, over a period of years.”

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As people rush to buy marijuana legally for the first time, their purchases will start to fill the state’s coffers with extra tax revenue. And some of this money will fund programs designed to offset the effects of increased pot use.

In Washington, the law that legalized marijuana provides much-needed funding for programs to educate the public about the health and social effects of marijuana, steer teenagers away from pot, and help people who become addicted to quit.

“Even though we know a lot from research on how to help young people avoid using drugs like marijuana, most of the programs we know are effective were not getting funded,” Roffman adds.

However, not everyone believes that programs designed to keep teenagers from lighting up will work.

“We know that prevention programs have, at best, mild marginal effects,” says Reuter. “These programs soothe the conscience of a government that’s a little uneasy about having done this.”

Marijuana legalization could also lead to changes that are not apparent right away.

“A lot depends on what marijuana use will substitute for,” says Reuter. “It is not as though there are more hours in the day. People are going to be stoned instead of something else.”

In some cases, increased pot smoking could end up being a good thing, especially if it replaces a night of heavy drinking.

“If young males reduce their binge drinking by twenty percent—an entirely made-up number—even if they increase their hours stoned by fifty percent, public health may be well ahead,” says Reuter.