THE BLOG

Interpreting Hazy Warnings About Pot and Mental Illness

11/17/2011 09:02 am ET

Smoking pot won't make you crazy, but trying to find the truth behind the
recent rash of headlines regarding a supposed link between cannabis and
mental illness might.

According to the Associated Press and other news sources, a new study
in the British medical
journal The Lancet reports that smoking cannabis ­- even occasionally ­- can
increase one's risk of becoming psychotic. It sounds alarming at first, but
a closer look at the evidence reveals that there's less here than the
headlines imply.

First, there is no new study. The paper published in The Lancet is a
meta-analysis -- a summary of seven studies that previously appeared in
other journals, including some that were published decades ago. Second, the
touted association between cannabis and mental illness is small--about the
same size as the link between head injury and psychosis. Finally, despite
what some new sources suggest, this association is hardly proof of a
cause-and-effect relationship between cannabis and psychosis,

So why the sudden fuss?

Part of the answer is political. The recently elected Conservative British
government longs to stiffen penalties against marijuana users. One way to
justify this move involves convincing the public that The Lancet proved that
puffing the weed will make you batty. Of course, that's not what the article
says at all.

In fact, investigators actually reported that cannabis use was associated
with a slight increase in psychotic outcomes. However, the authors
emphasized (even if many in the media did not) that this small association
does not reflect a causal relationship. Folks with psychoses use all
intoxicants more often than other people do, including alcohol and tobacco.

Cannabis use can correlate with mental illness for many reasons. People
often turn to cannabis to alleviate the symptoms of distress. A recent study performed in Germany showed that cannabis offsets certain
cognitive declines in schizophrenic patients. Another study shows that psychotic symptoms predict later use of cannabis, suggesting
that people might turn to the plant for help rather than become ill after
use.

Perhaps the most impressive evidence against the cause-and-effect
relationship concerns the unvarying rate of psychoses across different eras
and different countries. People are no more likely to be psychotic in Canada
or the United States (two nations where large percentages of citizens use
cannabis) than they are in Sweden or Japan (where self-reported marijuana
use is extremely low). Even after the enormous popularity of cannabis in the
1960s and 1970s, rates of psychotic disorders haven't increased.

Despite this evidence, we'd like to spread the word that cannabis is not for
everybody. Teens should avoid the plant. Folks with a predisposition for
mental illness should stay away, too. This potential for health risks in a
few people, however, does not justify criminal prohibitions for everyone.
(We wouldn't pass blanket prohibitions against alcohol simply to protect
pregnant women, for example.) The underground market does an extremely poor
job of keeping marijuana out of the hands of teens and others who should
stay away from it. A regulated market could better educate users to
potential risks and prohibit sales to young people.

Consequently, the review in The Lancet suggests that if cannabis really does
alter risk for mental illness, we can't leave control of sales to folks who
are willing to break the law. Instead, a taxed, regulated, age-restricted
market is our best chance to keep any negative consequences of marijuana
under control.