Teen pot use may cause `irreversible` decline in IQ

Washington - An international research team has found that the persistent, dependent use of marijuana before age 18 cause lasting harm to a person’s intelligence, attention and memory.

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 age 13 and age 38 IQ tests 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 organized 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 is led by Terrie Moffitt and Avshalom Caspi, psychologists who hold dual appointments at Duke and the Institute of Psychiatry at King’s College London.

About 5 percent 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 of the 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 that the 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.

“Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come,” Meier said.

The results appeared online in PNAS.