People who smoke marijuana on a regular basis for years and those who are dependent on it are significantly more likely to have economic and social problems at midlife than those who use it only occasionally or not at all, new research shows. And the longer that people regularly smoke, the greater their chances of having these troubles.
The study does not prove that marijuana causes these problems, but it does go further than probably any other research has done before to demonstrate a strong link. The paper was compiled from information gathered on nearly 1,000 New Zealanders in the town of Dunedin, who were checked on and interviewed regularly from birth to the age of 38. It was published online March 23 in the journal Clinical Psychological Science.
People in the study who smoked regularly, defined as at least four times per week over the course of several years, had significantly more economic problems, such as high levels of debt, poorer credit ratings, limits on cash flow and even difficulty paying for food and rent, says study author Magdalena Cerdá, a researcher at the University of California, Davis.
Moreover, they also were more likely to exhibit antisocial tendencies in the workplace, including such things as lying and engaging in arguments with co-workers. And they were more like to have conflicts in their intimate relationships, she adds.
Heavy smokers also ended up in a lower “social class” than their parents, Cerdá says. (Social class was defined as level of job specialty, with professionals like doctors and lawyers at the top and unskilled laborers at the bottom.) Meanwhile, those who didn’t regularly smoke ended up in a higher social class than their parents. While noteworthy, the finding raises some philosophical questions about what “social class” really means and what the value of such distinctions are.
Unlike many studies, this paper tried to control for a wide variety of potentially confounding factors, such as ethnicity, social class of origin, family history of substance dependence, low childhood self-control, childhood IQ, adolescent psychological problems like depression and motivation level at age 18. The researchers also controlled for the use of alcohol and other drugs. When people using these substances were excluded and only those using solely marijuana were considered, the link held up, the study found.
It should be noted, however, that the findings don’t apply to light or occasional smokers. And Igor Grant, a physician and researcher at UC San Diego who wasn’t involved in the paper, clarified that people who admitted smoking four times per week were probably more realistically smoking on a daily basis. Meanwhile, only a small fraction of marijuana users— around 9 percent, according to one large study—become dependent on it. And this number, as with just about everything involving marijuana, is a subject of controversy, with some saying it’s higher and others saying it’s lower or inappropriately measured in the first place.
The study participants were interviewed by researchers at ages 3, 5, 9, 11, 13, 15, 18, 21, 26, 32 and 38.
Researchers found that cannabis and alcohol dependence were both about equally linked with downward mobility, antisocial behavior in the workplace and relationship conflicts. But marijuana dependence appears to be linked even more strongly to financial problems than alcohol dependence is, Cerdá says.
Of course, alcohol dependence has much worse health effects than heavy cannabis use, and this study didn’t address physical health, Cerdá says.
As to whether these results could be unique to New Zealand, Ceradá says she doesn’t think so because “there have been multiple studies looking at this, and they’ve been pretty consistent.” That said, the levels of regular use and dependence—15 and 18 percent, respectively, among the New Zealand study participants—is higher than those found in the United States. But these rates may go up as marijuana is legalized in more and more states.
(The reason those numbers are different is because some people smoke regularly without being dependent, and some become dependent without smoking heavily, Cerdá says.)
There are, of course, limitations to this study. “It is difficult to definitely establish a causal relationship in studies of this sort, as the authors acknowledge,” says Wayne Hall, a researcher at the University of Queensland who wasn’t involved in the paper. “It may be that cannabis dependence is a marker of other risk factors for social and economic adversity. [But] this is less likely given that the effects persisted after controlling for plausible confounders and that much the same pattern was true of alcohol dependence” in the group, he adds.
One of the strengths of the study was to show that people generally began to have economic and social problems after smoking regularly and were not different beforehand as teens on measures of IQ, motivation, impulsivity or likelihood of using other drugs, Cerdá says.
However, Grant says that it’s possible there is something unique about the type of person who goes on to smoke regularly or become dependent, and some of the problems encountered may have to do with underlying personality differences that are difficult to measure—or psychological problems not yet manifest—as opposed to marijuana itself.
“It’s not the ordinary person who would use marijuana every day for years on end,” Grant says. “Whether marijuana caused these problems or these were people who were destined to have these problems anyway, I don’t think we can really figure out.”