Now that California has legalized marijuana for recreational use for adults 21 years old and older, one pressing public health question is whether legalization will contribute to greater use of cannabis among younger users, especially teenagers.
An editorial last year in the Journal of the American Medical Association asserted that, like alcohol and tobacco, "products that are legal only for adults inevitably find their way into the hands of adolescents...to a significant degree."
But because so few states have legalized marijuana for recreational use — and most very recently — experts say much more research needs to be done to know what its impact on teens will be. "Sound public policy should be based on data that are meticulously collected and thoughtfully analyzed," the JAMA editorial argued.
The available evidence from states that have legalized marijuana for medical or recreational usage does give some indication of its impact. It shows that loosening cannabis laws has not led to large increases in the drug's use among young people, and in some cases none at all.
Regardless of its overall impact, public health experts stress the importance of mounting effective campaigns to educate teens about the potential health effects of marijuana use, emphasizing that legalization does not mean that the drug is harmless, and that there may be penalties or repercussions in school or workplaces if they are caught using it.
In addition to California, seven other states (Alaska, Oregon, Washington, Colorado, Maine, Massachusetts and Nevada) and the District of Columbia have legalized marijuana for recreational use among adults. But many of those states have approved legalization in the last several years — Alaska's law was approved in November 2014, Oregon's in October 2015 and California's in 2016 — so it's too soon to tell how it may have affected young people.
"Concerns that increased adolescent marijuana use is an unintended effect of state medical marijuana laws seem unfounded," according to a 2015 study published in Lancet Psychiatry.
The evidence from Colorado and Washington, which became the first states to legalize the drug for recreational use when they did so in 2012, is mixed.
Surveys of 8th- and 10th-graders in Washington found those students were 2 percent and 4.1 percent, respectively, more likely to report using marijuana after the legalization vote than they were before it, according to a review of survey data published in JAMA Pediatrics in February 2017.
"It's not a huge increase, but of course any kind of increase — particularly in youth — is a concern," said Magdalena Cerda, the study's lead author and a professor in the UC Davis Department of Emergency Medicine.
In Colorado, however, Cerda and her colleagues found there was no change to rates of use or attitudes about harmfulness after legalization.
She said one potential reason for the difference in the two states is that Colorado had a robust medical marijuana industry prior to the legalization of recreational cannabis, whereas Washington did not.
It is possible that strictly controlled medical marijuana dispensaries in Colorado had already diverted large amounts of marijuana toward the general public and teens, and that outright legalization had little impact because cannabis was already easily available.
With its long-running and well-established medical marijuana industry, conditions in California prior to legalization more closely resembled those in Colorado, Cerda said. But whether that means California will similarly see no change to teens' use of marijuana is not clear.
"I don't think we can make those conclusions yet," Cerda said.
She added that there needs to be more research on changes in use and in attitudes about marijuana over longer periods of time and among larger groups of people.
Many more states have legalized marijuana for medical purposes than for recreational use. Since California became the first state to do so in 1996, 24 other states have followed suit.
As a result, there is much more research on the impact of marijuana use in those states. In general, the evidence does not show an increase in teenage marijuana use there.
An in-depth 2015 study published in Lancet Psychiatry, based on an annual survey of 8th-, 10th- and 12th-grade students administered in 400 schools nationwide, concluded that there is "no evidence for an increase in adolescent marijuana use after passage of state laws permitting use of marijuana for medical purposes."
"Concerns that increased adolescent marijuana use is an unintended effect of state medical marijuana laws seem unfounded," the researchers noted.
Marijuana use among 12-to-17-year-olds nationally has ticked up in recent years, after a long period of decline.
But in general, the increase has not come in states where marijuana has been legalized for medical uses. A study in the American Journal of Public Health showed that "passing medical marijuana laws does not seem to directly affect the views of young people in medical marijuana states."
Regardless of its legal status, though, there are clear health dangers from overuse of marijuana — and teenagers are at greater risk, according to a recent National Academy of Sciences report.
The report pointed to the impact of "persistent cannabis use" on cognitive functioning, and said it was associated with a decline in scores on IQ tests. Researchers said these findings point to marijuana having "a neurotoxic effect on the adolescent brain," underscoring the importance "of prevention and policy efforts targeting adolescents."
Both Cerda of UC Davis and Dr. Theodore Petti, a professor at Rutgers University's Robert Wood Johnson Medical School, said states that legalize marijuana should also bolster their education and prevention efforts.
Though they should not go overboard in dramatizing the risks of marijuana, Petti said, health officials must make clear to young people that the drug — like alcohol or tobacco — can have negative effects on health and learning.
"When teenagers understand the potential risk, and it's put to them not with drama but just the facts, teenagers will be more likely to say, 'Maybe this is not something I want to get engaged in,'" Petti said.