Good news for marijuana users who suffer from mental illness or have a propensity toward it: Adult use of the drug does not cause depression or anxiety, according to a new study.

DepressionThe report, published in February in the journal JAMA Psychiatry, found that smoking cannabis as an adult is not associated with any of a range of mood and anxiety disorders. That includes bipolar disorder, major depressive disorder, and any of a number of anxiety disorders.

JAMA Psychiatry is a journal of the American Medical Association. The researchers behind the study examined medical records from almost 35,000 American adults who took part in the National Epidemiologic Survey on Alcohol and Related Conditions.

The sample size is much larger than in most marijuana studies, meaning the results are more likely to be reliable. Scientists studied the prevalence of the subjects’ cannabis use between 2001 and 2002, then examined their rate of mental health disorders between 2004 and 2005.

“Cannabis use was not associated with increased risk for developing mood or anxiety disorders,” researchers said.

The study controlled for other factors that might contribute to either cannabis use or mental illness, such as family history, early childhood environment, socioeconomic conditions, and psychiatric history.

Contradicting earlier studies

Earlier studies have suggested that marijuana use is linked to anxiety and depression. The new survey refutes that conclusion, and with better numbers.

The study did have bad news, at least in terms of public perception. When the report was published, news stories quickly trumpeted its finding that there is a correlation between cannabis use and addiction to other drugs, including alcohol.

But the association doesn’t tell us much of anything. People who use pot may indeed be more likely to develop an addiction to alcohol or cocaine or heroin – not because one thing leads to the other, but because these people have a personal predisposition toward drug use and addiction of any kind.

This is not a hard idea to prove. People who smoke cigarettes are also more likely to drink alcohol regularly, but it’s well-established that tobacco doesn’t cause alcoholism. Instead, many smokers simply like getting a buzz, any buzz.

“The findings concerning cannabis raise the question of whether alcohol use also contributes to the risk of subsequent substance use disorders,” said lead author Mark Olfson of Columbia University. In other words, even if one drug causes addiction to other drugs, it might be the alcohol, not the marijuana.

Previous findings may have been due to uncontrolled factors

Brain, Mental IllnessOlfson said the mental health data are much more interesting, especially since they contradict previous results. The earlier findings, he said, may have been a result of uncontrolled factors.

Scientists try to use strict controls in their studies. These allow them to rule out external factors that could be causing particular results. For example, a control for family upbringing could allow researchers to conclude whether that or marijuana use is the real cause of addiction.

Keith Humphreys, a specialist in addiction and mental health medicine at Stanford University, called the study “a strike against the hypothesis that cannabis use causes mood and anxiety disorders.”

There is one flaw, he said, in that it doesn’t address previous research finding an association between cannabis use and schizophrenia. But the causal link between the two has never been fully explained. As with addiction, it’s possible the marijuana use causes schizophrenia, it’s possible schizophrenia leads to marijuana use, and it’s possible something else leads to both.

“I don’t know if we will ever know because it’s just hard to predict rare events, and schizophrenia is rare,” Humphreys said.

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