Teen pot use linked to later decline in IQ

A new, extensive study supports what addiction researchers have asserted for years: marijuana use is especially harmful to the developing brain and can cause irreversible damage. The study closely follows other new research that has found heavy diversion of so-called “medical marijuana” to youth.

The latest study — peer-reviewed and published in the Proceedings of the National Academy of Sciences — was funded with government grants from the United States, Britain and a foundation in Zurich.
The study examined survey data from more than 1,000 people in New Zealand during a year-long ending in 1973. As the Associated Press reports:

“In addition to IQ tests, they were interviewed five times between ages 18 and 38, including questions related to their marijuana use. At age 18, 52 participants indicated they had become dependent on marijuana, meaning that they continued to use it despite its causing significant health, social or legal problems. Ninety-two others reported dependence starting at a later age.
Researchers compared their IQ scores at age 13 to the score at age 38 and found a drop only in those who had become dependent by 18.”

The AP further reports:

“Among participants who’d been dependent at 18 and in at least one later survey, quitting didn’t remove the problem. IQ declines showed up even if they’d largely or entirely quit using pot at age 38, analysis showed.
The researchers got similar overall results for IQ decline when they compared participants who reported having used marijuana at least once a week on average for the past year. The researchers had no data on how much was used on each occasion or how potent it was.

“Dr. Duncan Clark, a researcher at the University of Pittsburgh, said he’s not convinced that mental decline is only in those who become dependent by age 18. He said the main lesson he sees in the overall study results is that to preserve one’s IQ, it’s best to avoid marijuana entirely, no matter what your age.

The researchers also surveyed people who knew the study participants well at age 38. They found that the more often participants were rated as marijuana-dependent in the surveys over their lifetimes, the more memory and attention problems were noticed by their acquaintances over the previous year.”

This study of New Zealanders follows research reported on in the August edition of the Journal of the American Academy of Child & Adolescent Psychiatry, which found heavy diversion of so-called “medical marijuana” to youth. Researchers at the University of Colorado Denver — including Dr. Christian Thurstone, who comments on the study here — found that of 164 teens in substance abuse treatment, nearly 74 percent reported they were using someone else’s medical marijuana.


Vote No on Question 3 in November 2012:  The proposed Massachusetts so-called medical marijuana law is too loose and ripe for abuse. Similar laws are failing in other states.  Informed voters are switching their votes to No.

Massachusetts doctors oppose Question 3, as do the Massachusetts Chiefs of Police.  This is not good public health policy.