Many studies have linked adolescent cannabis use and risk of becoming psychotic later in life. However, the question of whether cannabis use actually causes some people to become psychotic is a difficult one to answer and the evidence remains inconclusive. Although it is possible that cannabis use could be a direct cause of mental illness, it is also possible that both usage and mental illness are caused by an underlying third factor. In support of this latter theory, a recently published study comparing family history of mental illness in people with schizophrenia who either have or have not used cannabis, suggests that heavy cannabis use and risk of mental illness are both related to an underlying genetic predisposition. Furthermore, long-term studies on cannabis use have generally not considered that personality characteristics that have been linked to mental illness might also prompt a person’s decision to use drugs such as cannabis.
Cannabis, commonly known as marijuana and also by many other
names, is one of the most popular recreational drugs in the world. Although
many people try it without apparent ill effects, a large amount of evidence
exists that individuals who engage in heavy cannabis use before the age of 18
have an increased risk of developing a psychotic mental disorder later in life (McLaren, Silins, Hutchinson, Mattick, & Hall,
2010). Testing whether or not cannabis use actually causes people to
become psychotic is difficult to do, because the only truly conclusive way to
do so would involve experimenting on a bunch of people and randomly assigning them to either be users or non-users for a long period of time,
and assessing their mental health status before and after. Due to ethical
constraints this is not possible (especially in the current climate of
political correctness) so researchers have had to make do with alternative
study methods. The strongest evidence for a causal role of cannabis comes from
prospective cohort studies in which very large groups of people are initially
assessed in regards to their mental health and drug use, preferably in
adolescence, and then reassessed over a period of years. A review of 10 studies
involving seven cohorts in six different countries showed that all but one of
these studies found that there was an association between cannabis use and
later risk of psychosis (McLaren, et al., 2010).
For example, the first such study, and one the largest, assessed over 45,000
Swedish male conscripts aged 18 and then tracked how many were admitted to
hospital for schizophrenia over the next 15 years. This study found that those
who had used cannabis between 10 and 50 times before the age of 18 were three
times more likely to be hospitalised for schizophrenia compared to non-users,
while those who had used more than 50 times before age 18 had a six-fold risk
compared to non-users. Several other cohort studies also found that heavier
usage was associated with increased risk. To put this in perspective,
schizophrenia occurs in less than 1% of the general population (van Os & Kapur, 2009), so even if with
increased risk associated with heavy use, only a small minority of cannabis
users would be affected.
While these prospective studies are consistent with the idea
of cannabis causing later psychosis, insofar as usage preceded the development
of symptoms and heavier usage was associated with higher risk, alternative
explanations for the relationship between cannabis usage and psychosis cannot
be ruled out. It is also possible that people who are at greater risk of
becoming mentally ill are also more inclined towards cannabis usage, or that
there is some third factor underlying both. For example, a limitation of the
Swedish cohort study was that it did not assess whether participants who became
mentally ill had also used other drugs after the age of 18, such as
amphetamines, that might lead to psychotic symptoms (McLaren, Silins, Hutchinson, Mattick, & Hall, 2010).
Furthermore, cohort studies mostly have not considered genetic factors or
personality traits that are associated with heavy use of cannabis and with risk
of psychotic disorder.[1]
People who choose to engage in heavy cannabis usage might do so because they
have characteristics that also predispose them to eventually become psychotic.
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| Artistic view of how the world feels like with schizophrenia |
The authors of a recent study have argued that that the
genetic/familial risk of schizophrenia is what accounts for the association
between cannabis use and schizophrenia (Proal,
Fleming, Galvez-Buccollini, & DeLisi, 2014). That is, cannabis use
is probably not the cause of schizophrenia but itself a manifestation of the
underlying genetic factors that also case schizophrenia. This study compared
people diagnosed with schizophrenia, who either did or did not have a history
of heavy cannabis use in adolescence prior to the onset of their illness, with
two comparison groups of people who did not have schizophrenia and who either
did or did not have a history of heavy cannabis use in adolescence.[2]
They then determined how many people in each group had first degree relatives
who had been diagnosed with a psychiatric illness. This was done in order to
assess the genetic/familial risk of schizophrenia for members of each group. If
cannabis has a special causative effect in the development of psychosis that
goes beyond pre-existing genetic risk, then cannabis users who had become
psychotic would be expected to have a lower genetic risk compared to non-users
who had become psychotic. What the authors found was that, compared to the two
comparison groups, people with schizophrenia had the same elevated level of
familial/genetic risk regardless of whether they had or had not used cannabis.
Hence, they argued that it is the genetic risk that is most likely the cause of
schizophrenia, and that cannabis use is unlikely to be the cause, although they
admitted that it might hasten the onset of symptoms. Additionally, relatives of
the two cannabis-using groups, had high rates of drug use in general, which is
in line with research indicating that some people have a genetic predisposition
to use drugs.
As well as family history of mental illness, certain
psychological characteristics are associated with an increased risk of
developing psychotic symptoms. Specifically, some people who are not clinically
disturbed have certain tendencies that resemble mild psychotic symptoms, such
as experiencing unusual perceptions (e.g. feeling that strangers can read you
mind) and holding peculiar beliefs about the nature of reality (e.g. that
aliens are influencing events on earth). Psychologists refer to these
tendencies as schizotypy,
because of their resemblance to more extreme symptoms observed in people with
schizophrenia. People with high levels of schizotypy have an elevated risk of
becoming fully psychotic later in life, although it is important to note that
this only occurs in a minority of people with these tendencies[3]
(Chapman, Chapman, Kwapil, Eckblad, &
Zinser, 1994). Many research studies have found that heavy users of
cannabis also tend to be high in schizotypy traits compared to non-users (Fridberg, Vollmer, O'Donnell, & Skosnik, 2011).
Additionally, older users tend to have more severe schizotypy traits compared
to younger users, suggesting that symptoms might increase over time in users.
Once again, it is difficult to say whether having schizotypy traits predisposes
people to use cannabis, or whether using cannabis increases schizotypy. It is
also possible there could be a two-way relationship. However, there is some
evidence that schizotypy traits in heavy users tend to emerge before they first
start using. One survey of users tested this by asking participants who
indicated that they had schizotypy symptoms to estimate when they first noticed
them occurring, and to state when they first began to use cannabis (Schiffman, Nakamura, Earleywine, & LaBrie, 2005).
In the majority of cases, participants said that they had first noticed having
schizotypy symptoms a few years before ever using cannabis. Of course this does
not necessarily mean that schizotypy causes people to use cannabis, but it may
well be a factor. More to the point, it is possible that heavy cannabis users
may have an increased risk of psychosis because they naturally tend to be
higher in schizotypy traits, rather than because of their drug use. However, it
is also possible that the two act in combination and may be mutually
reinforcing.
Schizotypy also tends to be associated with other personality
traits that may be relevant to mental health, such as high neuroticism, and low
conscientiousness and agreeableness. Neuroticism has been identified as an
independent risk factor for schizophrenia in prospective studies (van Os & Jones, 2001) and for mental
disorders in general (Malouff, Thorsteinsson,
& Schutte, 2005). Although heavy cannabis users tend to be higher in
schizotypy than non-users, they do not tend to be higher on neuroticism, although
they do tend to be lower than non-users in conscientiousness and agreeableness,
as well as higher in openness
to experience (Fridberg, et al., 2011).
The majority of users, even heavy users, do not go on to develop psychotic
mental disorders, so perhaps there is a particular subset of users who are most
at risk. Fridberg et al. suggested that those who are not only high in
schizotypy but also high in neuroticism and openness to experience as well as
low in conscientiousness and agreeableness might be particularly vulnerable.
People who have a family history of mental illness would also be of particular
concern.
To reiterate, determining the nature of the causal
connection between cannabis use and psychosis is very difficult. It is possible
that cannabis use during adolescence has a direct causal role, perhaps due to
the drug’s influence on the developing brain. However, the reasons that a
person chooses to take up cannabis use in the first place may reflect
pre-existing risk factors for psychosis, such as such as genetic/familial risk
and schizotypy traits. Future research studies should take these factors into
account in order to better help identify individuals who may be at the greatest
risk of harm.
Footnotes
[1] One cohort study using a genetic test found that in heavy users with a specific genetic polymorphism had an increased risk of psychosis compared to heavy users without it (Caspi et al.). However, a later study failed to confirm this result (McLaren, et al., 2010a).
[2] The authors were careful to include only people who had not used any other illicit drugs.
[3] In fact, some people with schizotypal tendencies are otherwise well-adjusted. Schizotypy is often associated with creativity and professional artists and stand-up comedians tend to be high in these traits.
Image Credits
Mystic Weed Rose by MorbidKittyCorpse at Deviant Art
Schizophrenia/Internal Symmetry by Craig Finn courtesy of Wikimedia Commons
Use of these artworks is permitted by the Creative Commons Licence and does not imply endorsement by the artists.
© Scott McGreal. Please do not
reproduce without permission. Brief excerpts may be quoted as long as a link to
the original article is provided.
This article also appears on Psychology Today on
my blog Unique - Like
Everybody Else.
Posts I have written about
psychedelic drugs (which are not linked to psychosis)
References
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