Bellinda Kontominas Medical Reporter
PEOPLE who smoke cannabis regularly more than double their risk of developing psychotic illness later in life, according to research that calls for increased awareness of the dangers of the drug.
Researchers found that among all cannabis users, including social and habitual users, the lifetime risk of psychotic illness increased by 41 per cent.
More than one third of Australians over 14 years of age have smoked cannabis, or marijuana, at least once in their life and one in 20 have used the drug in the past week, according to figures on drug use from the Australian Institute of Health and Welfare. Of the 1.8 million Australians who have recently used cannabis, one in six use it every day and a further one in five use it every week.
The study found the increased risk for psychotic illness was relative to the dose. Those who smoked cannabis regularly were at an increased risk of between 50 per cent and 200 per cent of developing schizophrenia and disorders with symptoms including hallucinations or delusions. This also suggested that stopping cannabis use would decrease the risk, said the lead author, Dr Stanley Zammit, a psychiatrist from Cardiff University and Bristol University in Britain.
Previous studies have had trouble unravelling the link between cannabis use and psychotic disorder. Smoking the drug could be a symptom of psychotic illness, rather than a cause, the research found. The researchers re-examined data from 35 international studies on psychotic illness and cannabis use involving more than 100,000 participants. Factors such as pre-existing mental illness, the use of other illicit drugs, IQ and social class were filtered out of the sample to try to isolate the effect of cannabis.
Dr Zammit said there was now sufficient evidence to warn young people that using cannabis could increase their risk of developing psychotic illness later in life, despite a lack of evidence to confirm a cause-and-effect relationship.
The risk was relatively low but significant, he said.
One in 100 people had a chance of developing severe psychotic illness. That risk increased to 1.4 in 100 if they had ever smoked cannabis.
In an accompanying comment, Merete Nordentoft and Carsten Hjorthoej, of the department of Psychiatry at the Copenhagen University Hospital, said cannabis had long been considered a harmless drug and its potential long-term effects on psychosis had been overlooked. "There is a need to warn the public of these dangers, as well as establish treatment to help young, frequent cannabis users," they wrote.
Cannabis accounted for 45 per cent of hospital admissions due to drug-induced psychosis in 2003-04, according to a study published in the Australian Medical Journal. John Saunders, Professor of Alcohol and Drug studies at the University of Queensland, said the latest research strengthened the need for increased education on the dangers of cannabis.