People who start using marijuana before the age of 18 and keep using it for years can cause lasting harm to their intelligence, attention and memory, new research has found.
Those who did not start using pot until they were adults did not show similar mental declines.
The discovery was made by a team of researchers from New Zealand, Britain and the United States, analysing data from more than 1000 people enrolled in the Dunedin Multidisciplinary Health and Development Study, which followed around 1000 people born in Dunedin in 1972 and 1973 from birth to age 38.
The research was published today in the Proceedings of the National Academy of Sciences.
Those who started using cannabis in adolescence and used it for years afterward showed an average decline in IQ of 8 points when their IQ tests at age 13 and age 38 were compared.
Quitting pot did not appear to reverse the loss, lead researcher Madeline Meier, a post-doctoral researcher at Duke University, said.
The key variable was the age of onset for marijuana use and the brain's development, Meier said.
Those who did not take up pot until they were adults with fully-formed brains did not show similar mental declines.
Before age 18, the brain was still being organised and remodelled to become more efficient, and may be more vulnerable to damage from drugs.
"Marijuana is not harmless, particularly for adolescents," Meier said.
About 5% of the study group were considered marijuana-dependent, or were using more than once a week before age 18. A dependent user is one who keeps using despite significant health, social or family problems.
At age 38, all study participants were given a battery of psychological tests to assess memory, processing speed, reasoning and visual processing.
The people who used pot persistently as teens scored significantly worse on most of the tests.
Friends and relatives routinely interviewed as part of the study were more likely to report the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.
While 8 IQ points may not sound much on a scale where 100 was the mean, a loss from an IQ of 100 to 92 represented a drop from being in the 50th percentile to being in the 29th, Meier said.
Higher IQ correlated with higher education and income, better health and a longer life, she said.
"Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come."
Multidisciplinary study co-leader Terrie Moffitt said the decline in IQ among persistent cannabis users could not be explained by alcohol or other drug use or by having less education.
Laurence Steinberg, a Temple University psychologist who was not involved in the research, said the study was among the first to distinguish between cognitive problems a person might have had before taking up marijuana, and those that were apparently caused by the drug.
The results were consistent with what had been found in animal studies, but it had been difficult to measure in humans.
"The simple message is that substance use is not healthy for kids. That's true for tobacco, alcohol, and apparently for cannabis." Avshalom Caspi, multidisciplinary study co-leader said.
Dr Simon Adamson, a senior lecturer from the Otago University at Christchurch's National Addiction Centre, said the findings provided powerful new insights into something that had been of concern to the public and professionals.
New Zealand had a high rate of cannabis use by international standards, with at least some use the norm in adolescence and early adulthood.
"Clearly we must focus energy on reducing the prevalence of cannabis use in adolescence."