"This is not a study about the association between
cannabis and psychosis, but about the association between specific
patterns of cannabis use . . . and an earlier onset of psychotic
disorders," Dr. Marta Di Forti, who led the research at the
Institute of Psychiatry at Kings College, said in an email.
Among more than 400 people in South London admitted to hospitals
with a diagnosed psychotic episode, the study team found the
heaviest smokers of high-potency cannabis averaged about six years
younger than patients who had not been smoking pot.
Psychosis is a general term for a loss of reality, and is associated
with several psychiatric diseases, including schizophrenia and
Some previous research has suggested that using cannabis might
trigger psychosis in some people, especially those who may be
vulnerable because of a family history of related mental illnesses
or specific gene mutations.
But the evidence has been unclear. For example, one recent study
from the Netherlands found it's equally possible that people prone
to psychosis may be more likely to smoke pot, possibly as a way of
"self-medicating" (see Reuters Health article of December 25, 2012,
In the new study, published in the Schizophrenia Bulletin, the
researchers focused on patterns of cannabis use, gender and the
relationship of those factors to the timing of a first psychotic
Age is significant, Di Forti's team notes in their report, because
the teenage years and early twenties are a critical time for
professional and educational development, so experiencing an acute
psychotic episode for the first time early on may negatively affect
the "likelihood of achieving optimum level of function."
The researchers surveyed 410 patients between the ages of 18 and 65,
two thirds of them male, all of whom had a psychotic episode and
were admitted to in-patient psychiatric units.
The surveys asked about history of using tobacco, alcohol, cannabis
and other illicit drugs. They also recorded the potency of cannabis
used, characterizing low potency as "hash-type" and high potency as
In a previous study based on police seizures of marijuana in South
East London, skunk-type cannabis was found to contain 16 percent
THC, the active compound in cannabis, compared to 4 percent in the
The researchers found that males were more likely overall to use
cannabis and also had a younger age of onset of psychosis. The mean
age at the time of the first psychotic episode for male users of
cannabis was 26, and for female users was nearly 29. That compared
with nearly 30 years old for male non-users and 32 for female
They also found the patients who started using cannabis at age 15 or
younger preferentially smoked high-potency cannabis more often and
had an earlier onset of psychosis than those who started using
cannabis after age 15.
The earliest onset was seen
among those who used high-potency cannabis daily — on average their
first psychosis was 6 years earlier than for non-users.
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"This study adds to the literature on earlier age of onset for those
with significant exposure to cannabis," said Dr. Wilson Compton,
deputy director of the National Institute on Drug Abuse, who was not
involved in the study.
Compton cautioned that this study specifically focused on
patterns of cannabis use among people who all developed psychosis,
but did not compare them to users who did not experience psychosis.
"The thorny question is whether they might otherwise have developed
the disease or would have not had mental illness. It's a distinction
we haven't figured out yet," Compton said.
Di Forti emphasized that it is important to counsel educators and
parents about the risks of cannabis use in teenagers.
"I would try and understand why people use cannabis, what do they
get from it first and then engage them explaining how using daily,
choosing high potency type can cause harm to their brain and
increase risk of psychosis," she said.
It is still unclear whether there are safe levels of use for
cannabis, she added. "We know for instance that alcohol can be
highly toxic or damaging in the long term to health but that
sensible use of it causes no harm. We do not yet know enough about
safe use of cannabis and more research is needed," she said.
In counseling teenagers on cannabis use, Compton said, "Parents face
the challenge of keeping their children safe in many spheres.
Parents must establish a nurturing environment, appropriate
supervision and being aware of who their children associate with."
In light of recent changes in marijuana laws in Washington state and
Colorado, Compton said he thinks more research will be done on the
effects of marijuana on mental illness to understand the risks
associated with use.
"Even if marijuana is legal for adults, and if cannabinoids have
some legitimate medical purpose, that doesn't mean that they are
safe for all individuals," he said.
Schizophrenia Bulletin, online
Dec. 17, 2013.
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