People who use cannabis every day are nearly twice as likely to suffer psychotic symptoms than non-users, an Otago University study has found.
The research, by Professor David Fergusson and colleagues at the Christchurch School of Medicine and Health Sciences, is part of a longitudinal study which has followed 1000 people
for 25 years.
People were asked at ages 18, 21 and 25 whether they used cannabis. In a typical year about 4 to 5 per cent said they used cannabis every day and 50 to 60 per cent said they never used it.
Professor Fergusson said daily users suffered a clear increase in psychotic symptoms. They were between 1.6 and 1.8 times more likely to experience symptoms such as hearing voices others couldn't hear, to suffer paranoid thoughts and feel isolated from others. There were no differences between males and females.
Professor Fergusson said about six major longitudinal studies had reported a link between cannabis and psychotic symptoms but there had been doubts about whether it could be due to other factors, such as use of other drugs. Questions were also raised about whether people with psychotic symptoms were more likely to use cannabis to start with.
The new study, to be published in the international journal Addiction, had controlled for such factors and still found a clear association.
"These are not huge increases in risk and nor should they be because cannabis is by no means the only thing that will determine if you suffer these symptoms," he said.
Occasional cannabis smoking would not have any impact.
Professor Fergusson said debate around cannabis use had been polarised into two camps - those who believed it caused terrible harm and those who believed it was not harmful and should be legalised. "I think the truth lies somewhere in the middle."
Ross Bell, executive director of the New Zealand Drug Foundation, said the study reinforced the fact the issue was not black and white.
"This kind of research sends out a warning we need to take into account the harm of cannabis."
Cannabis use and its impact on mental health had been widely discussed as part of a health select committee inquiry in 2003.
United Future's support agreement with the Government rules out any changes to the legal status of cannabis.
