The researchers examined the records of nearly 35,000 U.S. adults who participated in the National Epidemiologic Survey on Alcohol and Related Conditions. They examined the prevalence of marijuana use among the study participants in 2001 and 2002, then checked on the participants' rates of mental-health problems three years later in 2004 and 2005.
After controlling for a variety of confounding factors, such as socio-demographic characteristics, family history and environment, and past and present psychiatric disorders, the study found that "cannabis use was not associated with increased risk for developing mood or anxiety disorders."
Don't break out the celebratory blunt just yet, though. The study did find an association between marijuana use and later substance-use disorders, such as abuse of and dependence on alcohol, tobacco, marijuana and other drugs. But this isn't necessarily surprising: It's fairly obvious that if you use a substance, you're putting yourself at risk of a substance-use disorder.
People who use one drug often use others — think of the classic beer-and-cigarette combo. This is as true of marijuana as it is of, say, alcohol. "The findings concerning cannabis raise the question of whether alcohol use also contributes to the risk of subsequent substance use disorders," lead author Mark Olfson of Columbia University said in an email. But that issue is beyond the scope of the current study, he added.
The findings on mental health are more interesting, given the conflicting picture portrayed by previous research. But Olfson and his colleagues think some prior evidence of links between marijuana and psychiatric disorders could be due more to confounding factors than anything else.
Olfson's research is "a strike against the hypothesis that cannabis uses causes mood and anxiety disorders," said Keith Humphreys, an addiction and mental-health specialist at Stanford University, in an email. He notes, however, that the new study does not address a previously observed link between heavy marijuana use and schizophrenia. But, he added, the causality of that connection is far from clear. "I don't know if we will ever know because it's just hard to predict rare events, and schizophrenia is rare," he said.
The new study adds to prior research discrediting the connection between marijuana and common mental-health disorders. And it's important, because much of the federal government's current literature on marijuana includes claims about links between marijuana and depression that are inaccurate in light of the latest findings.
For instance, the Drug Enforcement Administration makes these claims in its official fact sheet on marijuana. And in its 2014 publication, "The Dangers and Consequences of Marijuana Abuse," the DEA mentions "depression" no fewer than 14 times, claiming that pot is linked to depression among teens, adults and even dogs.
Given that these documents are used to inform policy at the federal level and below, it is crucial that they reflect the best, most accurate research. This is especially true given the rapidly changing marijuana-policy landscape today.