Young adults who use cannabis from an early age are three times more likely to suffer from psychotic symptoms, say researchers at Queensland Brain Institute and School of Population Health in the University of Queensland.
A study of more than 3,800 21-year-olds has revealed those who use cannabis for six or more years have a greater risk of developing psychotic disorders or the isolated symptoms of psychosis, such as hallucinations and delusions.
The study is based on a group of children born at Brisbane's Mater Hospital during the early 1980s. They have been followed-up for almost 30 years.
"This is the most convincing evidence yet that the earlier you use cannabis, the more likely you are to have symptoms of a psychotic illness," lead investigator Professor John McGrath said.
The research, published in the latest edition of Archives of General Psychiatry
, also included the results of 228 sets of siblings.
"We were able to look at the association between early cannabis use and later psychotic symptoms in siblings. We know they have the same mother, they most likely have the same father and, because they're close in age, they share common experiences, which allows us to get a sharper focus on the specific links between cannabis and psychosis - there is less background noise.
"Looking at siblings is a type of natural experiment - we found the same links within the siblings as we did in the entire sample. The younger you are when you started to use cannabis - the greater the risk of having psychotic symptoms at age 21. This finding makes the results even stronger," Professor McGrath said.
"The message for teenagers is: if they choose to use cannabis they have to understand there's a risk involved. Everyone takes risks every day - think of the sports we play or the way we drive - and people need to know that we now believe that early cannabis use is a risk for later psychotic illness."
Schizophrenia is a serious disorder that affects about 1 in a 100 Australians, and usually first presents in young adults.