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 The results chime with previous studies linking schizophrenia and
cannabis, but suggest the association may be due to common genes and
might not be a causal relationship where cannabis use leads to
increased schizophrenia risk.
Cannabis is the most widely used illicit drug in the world, and its
use is higher among people with schizophrenia than in the general
population.
"We know that cannabis increases the risk of schizophrenia. Our
study certainly does not rule this out, but it suggests that there
is likely to be an association in the other direction as well – that
a pre-disposition to schizophrenia also increases your likelihood of
cannabis use," said Robert Power, who led the study at the Institute
of Psychiatry at King's College London.
Schizophrenia is a common and severe psychiatric disorder that
affects around one in 100 people. People who use cannabis are about
twice as likely as those who do not to develop it.
The disorder typically begins in late adolescence or early adulthood
and its most common symptoms are disruptions in thinking, language
and perception. It often includes psychotic experiences, such as
hearing voices or delusions.
While the exact cause is unknown, research to date suggests a
combination of physical, genetic, psychological and environmental
factors can make people more likely to develop it.
Previous studies have found a number of genetic risk variants
associated with schizophrenia, each of which slightly increases a
person's risk of developing the condition.
Cannabis use has frequently been associated with it, but there is
much debate about whether this is because of a direct cause, or
whether there may be shared genes which predispose people to both
cannabis use and schizophrenia.
A study published in March 2011 found that people who use cannabis
in their youth dramatically increase their risk of psychotic
symptoms, and that continued use of the drug can raise the risk of
developing a psychotic disorder in later life.
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And earlier research found that young people who smoke cannabis for
six years or more are twice as likely to have psychotic episodes,
hallucinations or delusions.
This latest study, published in the journal Molecular Psychiatry on
Tuesday, included 2,082 healthy people of whom 1,011 had used
cannabis. Each participant's genetic risk profile - the number of
genes related to schizophrenia each of them carried - was measured.
The researchers found that people genetically predisposed to
schizophrenia were more likely to use cannabis, and to use it in
greater amounts than those who had no schizophrenia risk genes.
Power said the result "highlights the complex interactions between
genes and environments" when it comes to cannabis as a risk factor
for schizophrenia.
"Certain environmental risks, such as cannabis use, may be more
likely given an individual's innate behavior and personality, itself
influenced by their genetic make-up," he said, adding that this
finding was important to consider when calculating the economic and
health impact of cannabis.
(Editing by Alison Williams)
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