Marijuana use among college-age people is at the highest level in three decades and fewer think using it is harmful, according to researchers at the University of Michigan.

Months before Michigan voters will decide whether to legalize marijuana for recreational use, the annual study found marijuana use among the nation's 19-to-22-year-olds has increased gradually over the past decade as marijuana becomes more easily accessible and young people view the drug as less risky.

Researchers also found that youths who do not attend college are more likely to use marijuana. The study also surveyed other drug use among the age group and found non-medical use of prescription narcotic drugs was at its lowest since the late 1990s.

The federal National Institute on Drug Abuse paid for the survey, Monitoring the Future Panel Study.

"In this country, laws are changing, attitudes are changing, people are not perceiving use, even regular use, as dangerous as they used to," said John Schulenberg, the study's principal investigator and a psychology professor at the university.

"And this could be the problem. On this daily use, the scientific evidence is pretty clear that this gets in the way of things, and it can be associated with, if not contributing to, a decline in mental health.

"If one is involved in heavy use, and they continue with that," Schulenberg said, "then their health and wellness and happiness is probably not as high as those who do not use or do not continue to use."

In 2017, the year for which the study was done, 38 percent of full-time college students reported using marijuana at least once in the previous year, and 21 percent reported using it in the prior 30 days, up from 30 percent and 17 percent, respectively in 2006. Both percentages peaked in 2016, the highest found since 1987, and remained steady in 2017.

The study found that high school graduates who did not enroll full-time in two- or four-year colleges used marijuana at the highest levels since the 1980s. In 2017, their annual use was 41 percent; 28 percent reported using the drug in the previous 30 days.

The disparity between college students and noncollege young people increased when researchers looked at regular marijuana use, defined as having used on 20 or more occasions in the previous 30 days. For college students, the rate was 4.4 percent, down from a recent peak of 5.9 percent in 2014. Their noncollege counterparts were nearly three times as likely to use the drug regularly, at 13.2 percent — the highest level since 1980, when the study began, and doubling from 2006. More males than females also said they regularly used marijuana.

Schulenberg said researchers at the university are planning to further investigate the disparity in regular use among college students and noncollege users. Schulenberg said perhaps noncollege users are more likely to hang out with older people who may use marijuana, or have greater contact with people who have access to medical marijuana.

"We’ve got a zillion ideas," he said. "We just don’t know what the science is."

The study also found that in 2017, 27 percent of people aged 19 to 22 perceived regular use of marijuana as carrying great risk of harm, the lowest since 1980. That percentage had peaked at 75 percent in 1991, when marijuana use in this age range was at historic lows.

Kevin Sabet, a former drug policy adviser to presidents Clinton, Obama and George W. Bush, called the results alarming. He said the potency of today's marijuana is typically much higher than that of the 1960s and '70s, according to federal researchers, and that the "commercialization and glamorization" of marijuana has encouraged its use.

"We know low-potent marijuana increases mental illness risk, car crashes, IQ loss," said Sabet, who also serves as president of the group Smarter Approaches to Marijuana. "We don’t really know what high-potent marijuana can do, and I think this is very concerning."

Sheila Vakharia, policy manager for the Drug Policy Alliance, said the data does not show if young people using marijuana are experiencing negative effects from it. Some survey analyses report 90 percent of people who use the drug never develop a problem, though the National Institute on Drug Abuse reports that recent data suggests 30 percent of those who use marijuana may have some degree of marijuana use disorder.

"There's a difference between trying and experimenting, and actually having a problem," Vakharia said. "We don't know if these people are smoking weed and then missing class, slacking on assignments, and if starting on marijuana has impacted relationships with family and friends."

She said other studies show that for peopleunder 21, marijuana use has remained steady in states with legal recreational marijuana and that the U-M survey suggests the strength of the illicit marijuana market.

Josh Hovey, spokesman for the Coalition to Regulate Marijuana Like Alcohol, a group seeking to legalize recreational marijuana in Michigan, said the group's survey shows 50 percent of Michigan residents have used marijuana at some point in their lives.

"According to the National Institute on Drug Abuse's own research, marijuana is far less addictive than alcohol or tobacco," Hovey said. "Adults are free to consume those other products, that’s why we are asking voters to vote yes on Proposal 1 in November."

Healthy and Productive Michigan, an opposition committee formed to fight legalization, did not immediately respond to a request for comment.

The Monitoring the Future Study surveys high school students on drug abuse and follows them into adulthood. The study surveyed people by mail who were in college or not enrolled in college as of March 2017.

The study also examined college-age youths' use of other drugs, all of which remained steady or declined. Schulenberg said further study could determine if young people are substituting marijuana for other drugs.

In 2017, 18 percent of the age group reported using illicit drugs, down from 2014. Non-medical use of prescription narcotic drugs is at its lowest since the late 1990s, and cigarette smoking among college students reached a record low of 7.9 percent since 1980. Ecstasy use declined significantly from a year ago from 4.7 percent to 2.5 percent of college students and from 8.6 percent to 4.7 percent for noncollege users.

Compared to those who were not in college full time, college students were more likely to use nonmedical amphetamines and binge drink or have five or more drinks per occasion. Binge drinking has declined gradually to 33 percent among college students in 2017. One in six college students reported having 10 or more drinks in a row.

Schulenberg said drinking at that level can put people at risk of alcohol poisoning, which can be deadly, and inhibits cognitive functions, which can endanger the drinker and those around him or her.

"If you're trying to get a foothold on adulthood," Schulenberg said. "This is probably going to get in the way."

bnoble@detroitnews.com