AUSTRALIAN doctors and health researchers have called for an anti-tobacco smoking-style campaign on the dangers of cannabis after a study found a link between long-term use of the drug and a significant, and possibly irreversible, drop in intelligence.
The landmark study, the first to compare the IQ of users before they began smoking the drug and after prolonged intake, found cognitive decline was most pronounced in people who started using as teenagers.
A senior lecturer at the University of NSW's psychiatry department, Matthew Large, said the findings were particularly relevant to Australia, which had one of the highest rates of cannabis use in the world.
''Can we, as a country, afford to have a significant proportion of people becoming less intelligent?'' said Dr Large, who was not involved in the study.
The British research was based on data from a longitudinal New Zealand study, which has tracked the health of more than 1000 individuals born in the 1970s in Dunedin.
Participants had their IQ tested four times before age 14, and again at 38 in 2010-11, and were asked detailed questions about their cannabis use throughout adolescence and young adulthood.
The researchers found participants who smoked cannabis regularly, four days per week or more, for a year before at least three testing phases had the greatest decline in intelligence, a drop of six points on the IQ scale.
Those people who had never used the drug during the 20-year study had a slight increase in intelligence when tested as adults.
The neurological effect of cannabis, which could not be explained by recent cannabis use, addiction to tobacco, alcohol or other hard drugs and schizophrenia, were most noticeable in users who began smoking as teens. Many of these participants who began smoking as teens failed to regain their lost brain functions up to a year after they stopped taking the drug.
The authors, whose findings were published in the Proceedings of the National Academy of Sciences journal, said: ''One hypothesis is that cannabis use in adolescence causes brain changes that result in neuropsychological impairment.''
In all drug users, an intelligence drop could not be attributed to dropping out of school, as the effects were found in users who completed high school.
The director of the National Cannabis Prevention and Information Centre, Jan Copeland, said the study findings were as ''close as you can get'' to showing cannabis use caused deficits in intelligence.
''[The researchers] knew exactly what [the subjects'] cognitive functioning was before they used cannabis, they knew everything about their social, cultural and personal circumstances, then mapped the cognitive decline against their cannabis use over time,'' she said.
The deputy director of policy at the University of Queensland Centre for Clinical Research, Wayne Hall, said the findings added to the case for preventive public health education to reduce adolescent initiation and use.
Dr Large, whose research has found a link between cannabis use and early onset schizophrenia, said the dangers of cannabis use needed to be communicated to young teens. ''Most people when they first start smoking cannabis are handed their first joint by their best friend's older brother when they are 13 or 14 with no knowledge of its risks or benefits,'' he said.
Professor Copeland said the age of first use among the general Australian population was 18.5 years but agreed that continuing public health campaigns were needed.