Persistent, heavy use of marijuana is associated with economic and social problems in adults, according to an international study led by researchers at UC Davis.

The analysis was done on a group of more than 900 people whom researchers have followed since they were born almost 40 years ago.

In the study, the people who smoked marijuana four or more times per week over many years were much more likely to end up in a lower social class than their parents.

“By that we mean they ended up in jobs that were lower paying, they were less prestigious and they required less skills than jobs that their parents have,” said lead author Magdalena Cerda, an associate professor of emergency medicine at UC Davis.

Cerda and other co-authors interviewed stressed that their study was observational and could not prove that marijuana use caused these problems, but the researchers took into account many factors that could skew results, including any history of criminal conviction for marijuana use, history of antisocial behavior during adolescence, impulsivity problems, and lower IQ.

Terrie Moffitt, a professor in the department of psychology and neuroscience at Duke University, was a co-author of the study, published in the journal Clinical Psychological Science. She said that in the group, the longer people used marijuana, the more their credit ratings fell. “So they were at the point in their late 30s where they wouldn’t be able to get a mortgage or borrow money to start a small business,” she said. “So this is a pretty crippling level of financial difficulties.”

The team also looked closely at people’s job performance and relationship issues. Those who were long term, persistent users of marijuana, were more likely to have problems with coworkers and productivity problems, like calling in sick when they weren’t actually sick, Moffitt said.

In short, long-term, persistent marijuana use was “not safe” for the users in their study, Cerda said. Problems were comparable to those seen in people who are dependent on alcohol.

“We found that in the case of economic and social harms, cannabis was not safer than alcohol and, in fact, in the case of financial difficulties, cannabis was more harmful than alcohol.”

Cerda said she was surprised when they saw that heavy pot users also reported more domestic violence than those who were not heavy users. “We found that it was comparable to the effect of alcohol dependence on abuse,” she said.

The people followed are part of the Dunedin Longitudinal Study. Every child born in Dunedin, a city in New Zealand, between April, 1972 and March 1973 was enrolled and has been followed in three to six year intervals since. More than 1,000 papers have been published in the 40 years of follow up about everything from asthma to experience in the workforce. The group is mostly white, but of varying socio-economic levels that is similar to the general population of the U.S., Cerda said.

But critics say the study is inherently flawed, because all current recreational pot smokers in New Zealand — and in much of the U.S. — are people who are willing to break the law.

“It’s really not a good indicator of marijuana’s effects itself,” said Mason Tvert, communications director for the Marijuana Policy Project in Washington, D.C., “but more of who’s more likely to break the law and use marijuana.”

Magdalena Cerda was on KQED Forum Wednesday morning talking about the study, and at least one caller had the same reaction as Tvert. (Others had stronger reactions, you can listen if you want to hear more.)

But journalist David Downs, who writes Legalization Nation for the East Bay Express and described himself as supportive of updating both medicinal and recreational marijuana use laws, said he found the study “super fascinating.”

He said the study suggested a common sense approach.

“If you drink a lot, if you smoke a lot, if you do any mind-altering activity heavily every day, I imagine it’s going to alter a lot of people’s life courses,” Down said. “I don’t hear anybody saying, ‘Smoke pot every day, it’s going to turn out great.’ ”

(Several people who called or emailed Forum said they were daily users and their lives were fine.)

Professor Wayne Hall directs the Centre for Youth Substance Abuse Research at Australia’s University of Queensland. He was not involved with the study, but he said the findings about economic and social problems were consistent with other studies, including in the Netherlands where marijuana is “defecto legal.”

“It’s an important study,” he said and seemed especially interested by the comparison with alcohol use.

“It’s telling us what people don’t want to hear,” he said. “There are patterns with marijuana use that can provide the same social outcomes as patterns we see with drinking.”

To be clear, this study did not look at health effects, which the authors agreed are far worse for alcohol than for marijuana. And because alcohol is more widely used it is “still a bigger problem,” UC Davis’ Cerda said.

But the study comes as it’s widely expected that a ballot measure to approve adult use of marijuana will be before California voters in November.

The study authors said they took no position on whether recreational marijuana should be legal. Moffitt said that among the study authors, some used marijuana and some did not. But the study highlights the importance of investing in programs to prevent regular marijuana use and to treat addiction early.

“What we don’t know with legalization is if cannabis abuse will start to become more prevalent like alcohol abuse is,” said Moffitt. “That’s the $64 million question.”

  • Palo Jon

    Sounds like a political agenda at work: although the “study was observational and could not prove that marijuana use caused these problems” – i.e. it looked only at correlations – the study still reached the cause-and-effect conclusion that “in the case of financial difficulties, cannabis was more harmful than alcohol.” Also, although this may be reading in too much, the researchers seemed disappointed that they “could not prove that marijuana use caused these problems” despite their best efforts to do so. Trust no one and believe nothing you hear or read.

    • Habib Bundy

      Do the moon men in your head give you good advice?

    • Gertrude Stein

      I agree. It also overlooks the fact that marijuana had a worse social stigma 40 years ago than it does today. So people in the study may have limited or abstained from use if they identified with being middle and upper class. Also it does not seem to address whether heavy pot smokers were also heavy drinkers and drug takers. So, yeah, I call BS.

  • DWallace32342

    There exist many confounding factors for a study such as this:

    -This was heavy use for years under prohibition. For one, prohibition policies can steer some away from better employment for a number of reasons, not only those related to criminal justice. Concerns with potential drug testing, predjudice against those who use cannabis, etc.

    -Selecting cases based on having a “dependency diagnosis” presents major selection bias. Maybe the link had more to do with “people who allow themselves to be labelled cannabis dependent”.

    -Psycho-social deficits related to alcohol and tobacco use are being rewritten. Researchers are finding that they are more of a factor than previously thought. Any adjustment for these two confounders, which commonly exist for cannabis consumers, should be taken with a large grain of salt. It is quite possible that their effect was significantly underestimated.

    -Reverse causation was not examined. People who happen to have lower income and other problems tend to use more recreational substances.

    -Many people self-medicate a wide range of health conditions with cannabis, especially heavier consumers. Known or unknown mental and physical health issues can affect employment.

    Potential confounders are endless. As the authors state, “we do not purport to report a causal relationship between cannabis dependence and economic/social problems; cannabis dependence could be a marker of a life trajectory characterized by social and economic adversity”.

    This is not to say that cannabis cannot be a vice for some or lead to contentment with one’s life as is, but to say that usage, even regular usage, is particularly harmful is not justified by this study.

  • jeffJ1

    Can you really do an apples to apples comparison of outcomes among marijuana users in jurisdictions that treat the substance (and its users) so comically differently? In other words, will outcomes be comparable between someone from semi-rural New Zealand, someone from the Netherlands, and someone from New York City?

  • Roy Jordan

    Decriminalize marijuana laws this November in order to avoid being the next Federal Government prison income generator! Marijuana laws should have been changed years ago!

  • pilot210

    Seems like they are grinding an axe pretty thoroughly. I happen to know dozens of regular recreational consumers who are VERY successful systems designers in Silicon Valley, making top 5% money. They don’t let it interfere with work, some even claim it helps them concentrate. Not my thing, but you’ll never convince me it has impaired anything in their lives. Correlation does not prove causation, period. I’m sure you can find people who have let it interfere, just as you can find those who shouldn’t drink alcohol.

  • Diane

    As an ER Case Manager I notice a sharp uptake in people coming in for Cannabis Cyclic Vomiting Syndrome. They seem to be admitted for about 2 days of which insurance usually pays 1 day. It is driving up the cost of healthcare. I just want to say ‘get some pedialyte, put some capzasin cream on your tummy, stop smoking the stuff and don’t come to the ER.


Lisa Aliferis

Lisa Aliferis is the founding editor of KQED's State of Health blog. Since 2011, she's been writing and editing stories for the site. Before taking up blogging, she toiled for many years (more than we can count) producing health stories for television, including Dateline NBC and San Francisco's CBS affiliate, KPIX-TV. She also wrote up a handy guide to the Affordable Care Act, especially for Californians. Her work has been honored for many awards. Most recently she was a finalist for "Best Topical Reporting" from the Online News Association. You can follow her on Twitter: @laliferis

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