Cannabis-users are three times more likely to suffer psychotic disorders than siblings who do not use cannabis, a study of over 3,800 young people born at the Mater Hospital in Brisbane over 20 years ago has found.
The research team from the Queensland Brain Institute and School of Population Health has just published its findings in a special early-release article for the American journal, Archives of General Psychiatry (Vol. 67, No. 5, May 2010).
The study looked particularly at 21-year-olds who had been using cannabis for six years or more, that is, from the age of 14 or 15 years.
Although previous studies had shown a link between cannabis use and psychotic disorders (e.g., paranoia, schizophrenia, depression, delusions and hallucinations), critics have argued that there are other factors which could explain the results, including a predisposition to mental illness and environmental factors.
This study compared cannabis-users with siblings who had not used the drug, removing some of these factors.
“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.
“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.
“The message for teenagers is: if they choose to use cannabis they have to understand there’s a risk involved ï¿½… and people need to know that we now believe that early cannabis use is a risk for later psychotic illness,” Professor McGrath said.
The research confirms earlier Swedish and New Zealand studies published in the Lancet (1987) and the British Medical Journal (November 2002), respectively. The Swedish study found that heavy cannabis use at age 18 increased the risk of later schizophrenia six-fold. However, it was criticised because it did not establish whether adolescent cannabis use was a consequence of pre-existing psychotic symptoms rather than a cause.
The New Zealand study was based on an examination of over 1,000 people born in Dunedin in 1972-73, who were then subject to follow-up later in their lives. It found that, after making allowance for earlier psychotic illness, 10 per cent of youngsters who were cannabis-users by the age of 15, in the sample, developed schizophrenic illness by age 26 (compared with a mere 3 per cent incidence of schizophrenia among the rest of the population).
“Our findings suggest that cannabis use among psychologically vulnerable adolescents should be strongly discouraged by parents, teachers and health practitioners,” the New Zealand researchers said. (British Medical Journal, November 23, 2002).
Although it was not a central point of its research, the Queensland study showed the alarming extent of cannabis use among young Australians. Of the 3,800 young people followed up at the age of 14, 283 (7.9 per cent) reported using alcohol or illicit drugs. At the age of 21, 17.7 per cent reported using cannabis for three or fewer years, 16.2 per cent for 4 to 5 years, and 14.3 per cent used for 6 or more years.
Among those who had ever used cannabis, 11 per cent reported daily use, 13.8 per cent reported use “every few days”, and 22.6 per cent reported use “once or so per month”.
The results of the study add to the catalogue of other devastating effects of cannabis use.
The Drug Advisory Council of Australia (www.daca.org.au) has documented the damage which cannabis can cause.
Research from the Alfred Hospital in Melbourne has found that people who smoke cannabis risk lung disease 20 years earlier than those who smoke only cigarettes. Patients report symptoms from breathlessness to chest infection, which can commence as early as age 28. (ABC Radio News, March 25, 2006).
The Sutherland Hospital in Sydney conducts a cannabis clinic to treat more than 400 cannabis-users.
The New South Wales Government has confirmed that cannabis use can lead to psychosis, depression, anxiety, increased suicide, impaired cardiovascular, respiratory and immune systems, premature ageing, short-term memory dysfunction, slow brain development in young people, addiction and mental illness. (NSW Minister for Health’s media release, March 19, 2006).
Cannabis-users are also far more likely to be involved in motor vehicle accidents.
The Drug Advisory Council of Australia has been pressing governments to legislate for mandatory detoxification and rehabilitation of drug-users, in order to reduce the demand for illicit drugs, and to reduce usage and the associated crime.
Illicit drug use (largely cannabis, methamphetamine (“ice”), cocaine and heroin) is a major contributor to Australia’s crime rate.
John McGrath et al., “Association between cannabis use and psychosis-related outcomes using sibling pair analysis in a cohort of young adults”, Archives of General Psychiatry (American Medical Association), Vol. 67, No. 5, May 2010.