Cannabis causes psychotic disorders

AFA Report

A study of over 3,800 young people born at the Mater Hospital in Brisbane over 20 years ago has found that cannabis users are three times more likely to suffer psychotic disorders than siblings who do not use cannabis.

The research team from the Queensland Brain Institute and School of Population Health have just published their findings in the latest issue of the Archives of General Psychiatry (March 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.

Specific link

“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... 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 sixfold. However, it was criticised because it did not 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, and then subject to follow-up later in their lives.

It found that making allowance for earlier psychotic illness, 10 per cent of the cannabis users by age 15 in the sample (developed schizophrenic illness by age 26 compared with 3% 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, 23 November 2002)

Although it was not a central point in the study, 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%) reported using alcohol or illicit drugs. At the age of 21, 17.7% reported using cannabis for 3 or fewer years, 16.2% for 4 to 5 years, and 14.3% used for 6 or more years.

Among those who had ever used cannabis, 11% reported daily use, 13.8% reported use “every few days”, and 22.6% 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 ( 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 that only smoke cigarettes.

Patients report symptoms from breathlessness to chest infection, which can commence as early as age 28. (ABC Radio News, 25 March 2006)

The Sutherland Hospital in Sydney runs 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 aging, short term memory dysfunction, slow brain development in young people, addiction and mental illness. (NSW Minister for Health Media Release 19 March 2006)

Cannabis users are also far more likely to be involved in motor vehicle accidents.

The Drug Advisory Council has been pressing governments to legislate for mandatory detoxification and rehabilitation of drug users, to reduce the demand for illicit drugs, reduce usage and associated crime.

Illicit drug use (largely cannabis, methamphetamine (“ice”), cocaine and heroin) is a major contributor to Australia’s crime rate.