In a study involving more than 1,000 New Zealanders, individuals 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 38 were compared.
Quitting pot did not appear to reverse the loss either, said lead researcher Madeline Meier, a post-doctoral researcher at Duke University.
The key variable in this is the age of onset for marijuana use and the brain's development, Meier said.
Study subjects who didn't take up pot until they were adults with fully-formed brains did not show similar mental declines. Before age 18, however, the brain is still being organised and remodeled to become more efficient, she said, and may be more vulnerable to damage from drugs.
"Marijuana is not harmless, particularly for adolescents," said Meier, who produced this finding from the long term Dunedin Multidisciplinary Health and Development Study.
The study has followed a group of 1,037 children born in 1972-73 in Dunedin, New Zealand from birth to age 38 and was led by Terrie Moffitt and Avshalom Caspi, psychologists at Duke and the Institute of Psychiatry at King's College London.
About 5 per cent of the study group were considered marijuana-dependent, or were using more than once a week before age 18.
At age 38, all of the study participants were given a battery of psychological tests to assess memory, processing speed, reasoning and visual processing.
People who used pot persistently as teens scored significantly worse on most of the tests.
Persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.
The decline in IQ among persistent cannabis users could not be explained by alcohol or other drug use or by having less education, Moffitt said.
While 8 IQ points may not sound like a lot on a scale where 100 is the mean, a loss from an IQ of 100 to 92 represents a drop from being in the 50th percentile to being in the 29th, Meier said.
Higher IQ correlates with higher education and income, better health and a longer life, she said.
