Four teenagers spoke with The Oregonian about the possibility of legal recreational marijuana in Oregon (clockwise from upper left): Anna Del Savio, Brevin Bronton, Grace Ramstad and Julian Rist.

Grace Ramstad, an incoming junior at Centennial High School, dismissed the idea that legalizing marijuana would increase teen use of the drug.

"When people don't want to smoke weed, it's not really because it's illegal, it's for other issues," the Gresham 16-year-old said. "And when they do want to, they don't really care about whether or not it's illegal. You can see that through the amount of under-age drinking."

That was the unanimous opinion of a group of eight teens from around Multnomah County involved in leadership activities. They were gathered by The Oregonian to discuss Measure 91, which would legalize recreational marijuana for 21-and-over adults.

To this group of teens, at least, marijuana is such a common part of the landscape that it's hard for them to imagine that legalizing marijuana would change anything.

"The kids who are going to use it are already using it, whether it's legal or not" said Alex Zhang, 17, an incoming junior at Lincoln High School in Portland. He did concede that it might be easier for teens to get marijuana from older friends who could legally buy it – but he said it's not hard to find it now.

For all their nonchalance, however, teen use of marijuana is one of the central issues in the developing campaign over legalization.

Both sides argue heatedly over whether or not greater social and legal acceptance of marijuana will inevitably spark more teen usage – and whether youths will someday be bombarded with the cannabis version of the beer industry's babes-and-parties marketing.

Surveys already indicate that an increasing percentage of teens don't see marijuana as particularly dangerous, despite a growing number of studies raising concerns that the drug can have a harmful effect on brain development among youths. Researchers say their concerns have become heightened as more potent strains of marijuana became commonplace.

Krista Lisdahl, a brain researcher at the University of Wisconsin-Milwaukee, said brain-imaging studies of teens and young adults who smoke marijuana regularly show abnormalities in parts of the brain affecting intelligence.

"It needs to be said that regular cannabis use," she said in a study, "...may result in addiction and neurocognitive damage, especially in youth."

Drug abuse officials have also widely cited a National Academy of Sciences study showing an average drop of 8 points in IQ following heavy use of marijuana in teen years. And the Centers for Disease Control and Prevention found that two-thirds of students who received mostly Ds and Fs were heavy marijuana users.

"The potential Achilles heel for marijuana legalization is the impact on young people," said Mike Roach, a Portland clothing store owner who helped form the Health Action Network at Lincoln High School after being concerned about drug and alcohol use there.

"And when they do want to, they don't really care about whether or not it's illegal," says Grace Ramstad, one of the teens in leadership roles The Oregonian gathered to discuss legalizing marijuana.

Roach, who has been involved in a number of liberal causes, said legalization might someday make sense. But he said there are so many unanswered questions now that he'd rather let Washington and Colorado, both of which in 2012 became the first states to legalize recreational marijuana, act as pioneers.

Oregon, by some measures, has one of the highest rates of teen marijuana use in the nation. The Oregon Healthy Kids Survey from 2013 showed that 20.9 percent of 11th graders used marijuana in the last 30 days, as did 9.7 percent of eighth graders. Those rates are lower than they were in the early 2000s, but also a point or two higher than they were in 2006 and 2007.

Sponsors drafted the Oregon legalization measure with an eye toward reassuring voters who may be worried about the impact on kids. They've reserved 20 percent of the proceeds from marijuana taxes for prevention and treatment programs and would give the Oregon Liquor Control Commission broad authority to crack down on retailers selling to minors.

"People often confuse prohibition with control," said Amanda Reiman, who works for the group that helped write the Oregon measure – the Drug Policy Alliance -- and lectures at the University of California at Berkley on drug and alcohol policy.

Reiman argued that a tightly controlled retail environment, realistic education programs and more funding for treatment is the best approach for reducing youth consumption.

"We have an opportunity to create a responsible industry," she said, "and we're taking that very seriously."

"The kids who are going to use it are already using it, whether it's legal or not" said Alex Zhang, an incoming junior at Lincoln High School. The Oregonian gathered teens in leadership positions to discuss the measure that would legalize marijuana.

Roger Roffman, a retired University of Washington professor studied marijuana policy for several years after having dependence problems on his own.

"I'm not one to understate or underestimate the potential harm from marijuana use," said Roffman, "particularly for people whose brains are still developing."

Nevertheless, Roffman became a sponsor of the Washington legalization measure after being convinced that the status quo wasn't working.

"I think a realist would agree that protecting public health and safety has not gone very well under criminalization," said Roffman, author of "Marijuana Nation."

Kevin Sabet, a former White House drug adviser who has become the nation's most prominent opponent of legalization, said he supports better treatment and prevention efforts and a more humane approach to marijuana crimes.

But he argued that a legalized marijuana industry would inevitably turn toward marketing and advertising to minors.

"There's only one way to get your best customers," he said, "and that is to make sure they start young."

He argued that the last several decades have shown that use is affected by social acceptance of the drug.

Surveys show that marijuana use rose sharply among youths in the 1970s as it became a mainstream part of American culture, dropped during the "just say no" era of the 1980s and began to climb again over the last two decades.

"What that tells us is that social messages are important," said Sabet, adding, "Kids don't live in a bubble – when something is a badge of adulthood, like beer, that is very attractive to them."

What's not clear, however, is the impact of changing legal approaches to marijuana. Since 1996, 23 states and the District of Columbia have allowed medical marijuana, providing something of a test case for legalization.

As it happens, those states also tend to have higher youth rates of marijuana, according to data from the National Survey on Drug Use and Health. The 2011-12 survey found that just under 10 percent of Oregon teens aged 12 to 17 reported using marijuana in the last month – the sixth highest rate in the country.

However, one recent study concluded that the passage of medical marijuana laws didn't lead to increased use of marijuana among high school students. Instead, the researchers said, it may be that states that approved medical marijuana already had a social climate more forgiving of the drug.

Reiman, the Drug Policy Alliance official, pointed to another factor. Teens in states with harsher marijuana laws may simply be less willing to tell surveyors that they smoke pot, she said.

Marijuana legalization advocates also look to the experience in Colorado, where the state's main study of youth drug use showed a small drop in marijuana use from 2011 to 2013, which covers the period of the 2012 legalization vote. Officials cautioned that the drop was within the statistical margin of error – but in any case there certainly didn't seem to be any upward spike in use.

Bettina Friese, who has researched youth marijuana use for the Pacific Institute for Research and Evaluation in Oakland, Calif., said that result might have been encouraging. But she said that "I don't think anybody knows at this point" what would be the long-term impact of legalization.

She said it could take years before anyone knows for sure the impact legalizing recreational marijuana for adults. In the meantime, she said, she hopes policymakers intensify their focus on discouraging teens from using marijuana.

"We've been fairly successful at changing the norms around tobacco," she said, "so it's important we do the same around marijuana."

Since the 2012 vote in Colorado and Washington, officials have scrambled to show they are dealing with concerns about youth usage.

Colorado has launched a $2 million campaign trying to send the message to youths that they shouldn't be smoking pot as scientists are learning more about potential brain damage. The tagline: "Don't be a lab rat."

In Washington, Gov. Jay Inslee in June announced a $400,000 ad campaign encouraging parents to talk to their children about marijuana. And state officials announced new rules requiring stricter labeling requirements on edible marijuana products "to ensure they are not especially appealing to children."

The concern over edible products was raised by the well-publicized death of a 19-year-old Wyoming college student visiting Denver who leaped off a balcony after eating a pot-laced cookie.

The teens in The Oregonian's informal focus group nodded knowingly when they discussed the Colorado death.

Juan Perez-Torres, 17, an incoming Gresham High School senior, said he learned the dangers of edible marijuana products when three or four kids from his school went to the hospital after overdosing on that form of the drug.

Perez-Torres and the other teens say they realize that marijuana is something they'll have to figure out in their lives, just as they do alcohol and the array of other mind-altering substances.

"There is the common knowledge that things that are dangerous for you can still be legal," added Ramstad, the Centennial High student, citing tobacco as a leading example. "So there's not such an absolute concept that what is safe is legal."

-- Jeff Mapes