Twenty years ago, marijuana was America’s unwanted stepchild and illegal on every level.

Then, in 1996, California passed Proposition 215, the first to make medical marijuana legal statewide.

Today, 25 states, plus the District of Columbia and Guam, have legalized some form of medical marijuana.

Four states — Washington, Colorado, Oregon, and Alaska — have legalized the drug for recreational use for people over 21. Five more states will vote on the issue this November.

With the majority of Americans now in favor of legalizing marijuana, the trend seems inevitable.

Yet there are still controversies over health, and the legal and ethical implications of the drug for adult use. But when kids enter the picture, it becomes a lot clearer.

Advocates on both sides of the debate share concerns about teenagers using drugs. This raises an important question: How will marijuana legalization impact teen use rates?

To tackle this question, a group of researchers in Colorado designed a survey to examine marijuana use patterns among the state’s middle and high school students.

Read more: Marijuana use has doubled in U.S. since 2001 »

The Healthy Kids Colorado survey

The Colorado Department of Public Health and Environment (CDPHE) partnered with the University of Colorado, Denver, to form a comprehensive health survey for state teens in 2013, to be repeated every two years.

The surveys gathered data from more than 40,000 students at more than 200 middle and high schools, asking a range of questions about physical and mental health, substance use, and sexual orientation.

Since Colorado’s voters legalized recreational marijuana in 2014, the survey’s 2013 and 2015 data captured a snapshot into the short-term impact of legalization on these teenagers.

In particular, the survey examined whether the percentage of teenagers who were current users of marijuana — defined as having used within the 30 days before the survey was conducted — had changed.

In 2013, that number was 19.7 percent, compared to 21.2 percent in 2015.

These findings are consistent with national data that revealed teen use rates haven’t risen in states that have legalized medical marijuana.

“Marijuana use has remained relatively unchanged,” explained Leo Kattari, administrator of the survey, and Healthy Kids Colorado Survey coordinator at the CDPHE, in an interview with Healthline. “Among Colorado high school students, over the years 2005 to 2015, estimates of current marijuana use have fluctuated between 19.7 percent and 24.8 percent. None of these estimates are statistically different from each other.”

There are some possible limitations to these results.

A survey isn’t the ideal way to gather data. There are a lot of factors that can skew the findings.

For example, a low percentage of students responded to the survey. Those who did respond were exclusively students enrolled in school, meaning that teenagers not in school weren’t surveyed.

And the scientists had to depend on the teenagers’ ability to accurately recall and report their own experiences. Eyewitness testimony is notoriously unreliable for all age groups.

“As with any research, there are some limitations,” Kattari acknowledged. “However, administering the survey using the same methods year over year accurately provides data that can be compared to the previous years. Thus, we have a consistent estimate to measure changes over time.”

“This may not be strong data, but it is consistent with the lack of increases,” confirmed Dr. Wilson Compton, deputy director of the National Institute on Drug Abuse (NIDA), in an interview with Healthline. “We are not seeing in this survey a marked increase in drug use by teens, and that’s good news.”

Compton, however, thinks it’s a bit premature to be drawing conclusions from such recent data.

“It’s a little early to look at the impact of full legalization,” he said. “It only happened two years ago, and this data is mostly from last year. So, it takes some time for these big trends in social norms to wind their way through the system and to understand what the impact will be on youth drug use.”

Read more: Teens using e-cigarette devices to smoke marijuana »

This is your brain on puberty

Emerging neuroscience from the past 10 years has found increasing reason to believe that using marijuana as a teenager may have undesired consequences.

During adolescence, the human brain undergoes a large amount of construction work, pruning away neural connections that it isn’t using, and rediverting those resources into beefing up the connections that are used the most.

This process requires a complex and delicate dance of signaling chemicals in the brain to tell its nerve cells where to wire up, and how.

One of these chemical guideposts is endocannabinoid, a naturally occurring neurotransmitter in the brain.

As the name implies, the brain’s endocannabinoid chemicals are similar to the cannabinoid chemicals found in marijuana, such as tetrahydrocannabinol (THC) or cannabidiol (CBD).

Marijuana hits the brain with a hefty dose of these impostors, washing away the brain’s own chemical signposts. And when the outside cannabinoids take their place, they don’t necessarily do the same job as well. With incorrect directions, the brain’s nerve cells can’t wire up as efficiently.

While the resulting brain wiring differences are subtle, they’re also cumulative. A lifetime of use can result in impaired verbal abilities.

There may also be additional risk for teenagers who carry a predisposition towards certain mental illnesses involving miswiring of the brain, such as schizophrenia.

“This is the adolescent brain, [which] has plenty of stresses and plenty of work to do on its own,” Compton pointed out. “And it doesn’t need alcohol, tobacco, marijuana, and other substances to interfere with the normal and healthy development of the brain. So anything that helps reduce those risks, I think is a good thing.”

Relative risk is one of the questions on the table as alcohol and tobacco enter the equation.

“We should also consider whether a reduction in cannabis use is the best outcome we can hope for from teens,” suggested Amanda Reiman, Ph.D., manager of marijuana law and policy for the Drug Policy Alliance, and a lecturer at the University of California, Berkeley, in an interview with Healthline. “If cannabis use remains stable or even increases slightly, but alcohol use goes down and therefore so do car accidents, unplanned pregnancies, and assaults, then, in my view, legalization of cannabis is a huge success.”

She did add, “However, we do not advocate cannabis use for teens, and support delaying any cannabis use until the person is at least over 21.”

Compton is skeptical of this notion.

“We all are concerned about alcohol-related injuries and alcohol-related problems. So something that can reduce alcohol-related problems, we would all be in favor of,” he said. “What we don’t see is any evidence to suggest that, if marijuana [use] goes up, alcohol [use] goes down. There’s no evidence to suggest that these are countervailing or opposite trends. Much more likely is that we see them going hand in hand, so that as one goes up, the other tends to go up. The combination is particularly concerning and particularly dangerous.”

Currently, there isn’t a whole lot of reliable data on this topic. That’s why NIDA is launching a large-scale study of adolescent growth and development called the Adolescent Brain Cognitive Development Study (ABCD Study). This will follow 10,000 children from the ages of 9 to 10 into young adulthood, scanning their brains every few years to track changes as they deal with the stresses of growing up.

“We’re not going to be able to look at marijuana separately from the exposure to alcohol or tobacco, because so many of the kids that use one, also use the other substances,” explained Compton. “So, we will examine the impact of combination, and hopefully get some information about the influence of each substance separately. But it’s really the combination that most kids are experiencing and exposed to.”

This study may yield information that policymakers, parents, and adolescents can use when making their decisions.

Learn more: Cannabis during pregnancy impairs baby's brain development »

Teach your children well

Today, most kids from all walks of life are getting their information about drugs online, not from educators or parents.

One such online resource is ProCon.org, which ranks highly on Google searches for phrases like “marijuana controversy.”

According to Kamy Akhavan, president and managing editor of ProCon, they get visits from more than 25 million people annually from 87 countries.

About 40 percent of their audience is students, he reports, half of who are in high school. A quarter of them are in elementary or middle school, and a quarter are in a college or university.

Students and other adults under the age of 30 make up 60 to 65 percent of the total audience, he says.

“No one wants to be told how or what to think,” Akhavan told Healthline. “Providing the pros and cons in an unbiased and trustworthy manner is critical to helping people understand the issue. In terms of the marijuana debate, there are a lot of conflicting messages and opinions. Our only goal, really, is just to get people thinking about both sides of it. We want people to have something they can trust, for folks to be able to see what the best research is so they can make informed judgments for themselves.”

In surveys that ProCon has conducted of its readers, 35 percent of users said that they changed their opinion on a topic because of various expert discussions that they had read on the website.

“There’s a real power in being able to see smart people explaining their point of view, even if it’s completely contrary to your own,” explained Akhavan. “I think reasonable people will look at that and say, ‘You know what, that’s a really interesting point, I had not considered that.’”

He added, “There are often competing perspectives on these divisive issues. When we can’t even have political conversations at the Thanksgiving table with our families, how are we going to function as a society where we have to talk about controversy in order to solve these really big problems?”

Read more: Changes in marijuana policy unlikely to have real effects »

Give the kid a break

Rather like abstinence-only education, scare tactics haven’t worked on the latest generation.

The percentage of teenagers who don’t see a problem with marijuana use is at a new high. About 75 percent of 12- to 17-year-olds surveyed in 2013 reported they saw no great risk with using marijuana once a month, and 60 percent said that they saw no such problem with using it once or twice a week.

Instead of trying to wage a war of misinformation, Compton suggests another strategy to help at-risk kids avoid using marijuana and other drugs.

Get families involved.

“Some specific interventions can be useful in preventing the onset of marijuana [use], [as well as] helping kids generally stay healthier and be more likely to succeed as they grow and develop,” he said. “There have been family-based interventions that strengthen families and help them provide more supportive environments for teenagers and better supervision of teenagers. That’s just one example of an approach that’s been proven to be useful.”

In particular, Compton is worried about kids beginning to use marijuana at an early age. Early use, for many adolescents, can predict heavy or frequent use in the future.

“Most of the data suggests that the highest risk are those that start using early,” he explained. “So when a child starts at age 14 or 15, that’s a warning sign and a concern. Once you start not engaging in healthy behavior, there’s a possibility that will expand and continue.”

Reiman agrees that later is better.

“With cannabis, the focus should be on delaying use as long as possible,” she said.

Reiman would also like to see public awareness education keep pace with the rate of legalization.

“I think that cannabis education should take a page from the anti-tobacco campaigns, including education and taxation,” she suggested. “We have been hugely successful at reducing smoking rates among teens without making cigarettes illegal. We tax them and educate young people about their harms. We should apply this same model to cannabis education.”

So far, anyway, Colorado’s kids seem to be doing all right.

“Generally, young people in Colorado are making healthy choices,” Kattari concluded. “This is amplified when a young person has a trusted adult in their lives.”

Keep reading: Will marijuana follow in the footsteps of big tobacco? »