"Pre-legalization about 54 percent of 12-17 year-olds in Colorado reported great harm with regular marijuana use, and now post-legalization that's dropped to about 34 percent," he said.
"We're clearly seeing a significant decrease in the perceived harmfulness of marijuana, especially among young people."
And the data seems to match what he's seen among the real live teens in his clinic as well.
"We're seeing teenagers who are telling me, 'Why would I stop using marijuana? I don't believe it's addictive, I don't believe it has any bad effects, in fact it's my medicine for my anger, depression, anxiety or ADHD.'"
This year, Denver Public Schools (DPS) created a position for substance prevention. Michel Holien, the new supervisor, said that while DPS hasn't necessarily seen a sharp increase in marijuana use, they have noticed the shifting perceptions towards more accepting attitudes, and are working to combat them as early as the middle school level.
In the notoriously more-hippie-than-Denver city of Boulder, Colo., marijuana has long been entrenched in the culture, even before its legalization.
Father Peter Mussett, who serves as pastor at the Catholic Church on the campus of CU Boulder, said that even before legalization, 40 percent of incoming freshman were reporting use of marijuana on at least a monthly basis.
And while legalization has opened up more opportunities for conversations about marijuana, Fr. Mussett said it's also sparked more curiosity about the drug in more people than when it was still illegal.
"The curiosity is something that I find is one of the most poisonous parts of the legalization of marijuana because I think ultimately it's a toxic experience," he said.
"(Legalization) just encourages the culture to say getting high is a great thing, and getting high is not a great thing. Getting high is destructive, and you can come in with all the best intentions, and on the other side of it, it always ruins people's lives, continuously. It makes them dependent in worldy ways, and it does not actually encourage a good spiritual life surrendered to God."
Health risks of marijuana
(Story continues below)
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The biggest health concern for young people using marijuana is its harmful effect on the brain, which continues its development well into a person's 20s, Dr. Thurstone said.
The main active ingredient in marijuana, THC, binds to receptors in the brain and can cause a significant decrease in IQ over time. A 2012 study published in the National Academy of Sciences found that adolescent exposure to marijuana can lead to an 8-point drop in IQ, on par with the drop seen in children exposed to lead.
Another concerning impact is the relationship between adolescent marijuana use and schizophrenia. A study repeated by multiple research groups has found that adolescent marijuana use can quadruple a teen's risk of developing schizophrenia, Dr. Thurstone said.
Marijuana can also be addictive, with one in six adolescent users developing a dependence over time, despite the perceptions to the contrary.
"In the scientific and medical community there's not debate about that anymore," he said. "Marijuana is not just psychologically addictive but physically addictive."
A secondary health risk of marijuana use in adolescents is car accidents. The leading cause of death of 15-20 year-olds is automobile accidents, and the number traffic fatalities in which adolescents testing positive for marijuana spiked in Colo. after the surge of medical marijuana in the state after 2009.