THE link between the use of cannabis and the onset of psychotic illness has been further clarified in the largest study of its kind.
There has long been an association between cannabis use and age at onset of psychosis; however the nature of that association remains unclear.
Researchers have yet to conclude whether cannabis is causal in its association or not, or whether other factors such as age or gender better explain the association.
A team from WA Health services, University of Western Australia and the University of Melbourne investigated whether the age of first cannabis use brings forward the age at onset of psychosis.
Dr Brian Power from the South Metropolitan Heath Service says they would expect to “see a temporal relationship between the age at which one starts using cannabis, and age at onset of psychosis”.
Few studies have addressed this question with a sufficiently large sample size. This study recruited people from across seven sites in five states of Australia.
Of 7,955 people who screened positive for psychosis, 1,825 were randomly selected for interview using the Diagnostic Interview for Psychosis (DIP), a standardised semi-structured interview and assessment.
Out of this 997 people reported use of cannabis and were selected for further study.
Age at initiation of cannabis use and the age at onset of psychosis were directly and linearly associated.
There was an average delay of 7-8 years from first exposure to cannabis and the onset of psychotic disorders.
Much research has focused on adolescence as a particularly vulnerable period of brain maturation for those exposed to cannabis however Dr Powers says findings do not support the "window of vulnerability" hypothesis of schizophrenia.
“Those patients who used cannabis at age 12, had a similar trajectory of illness to those who were first exposed to cannabis at 19,” he says.
The reasons why earlier cannabis use may be more harmful are unclear and no robust evidence yet exists.
“The results suggest that cannabis may exert a cumulative toxic effect for those on the pathway to develop a psychotic illness, like schizophrenia,” Dr Powers says.
While mechanisms by which cannabis exerts such delayed effects are unclear, there is evidence that it may be due to sensitisation of the mesolimbic dopaminergic, or “reward” system, triggered by repeated cannabis stimulation to which susceptible individuals may be more sensitive.
Dr Powers says further studies are required before formulating public health measures.
“Although if we were better able to identify those young people at ultra high risk of developing schizophrenia, we could target cannabis use in these people, as potentially modifiable factors in their disease course.”