Is Cannabis (Marijuana, THC) Use Harming Brain Function?

marijuanaWith legalization in many states, including Oregon, more and more adolescents believe that regular use of marijuana is harmless and more now are using it daily.

In the study “Persistent cannabis users show neuropsychological decline from childhood to midlife,” (which you can read here… ) neuropsychological declines were significant and worse for those who started cannabis use as adolescents.

This study was prospective, following over 1000 individuals born in 1972/1973 to the age of 38 with interviews at ages 18, 21, 26, 32, and 38. Neuropsychological testing was done at age 13 before any marijuana use and again at age 38.

Here are some of the findings:

  • If you never used THC your IQ was unchanged to slightly improved = 100.
  • If you used THC regularly your IQ dropped, and if it was persistent use, IQ dropped 6 points.
  • There were significantly more memory and attention problems in persistent users.
  • There was greater IQ decline in those who started THC use as teenagers.
  • Greatest decline was in users of over 20 years, especially if they started THC use as adolescents.
  • This study ruled out pre-existing neuropsychological issues and education differences.
  • The impairment was global across 5 different domains of function and testing.
  • Others in the users life verified the impairment.
  • The impairment was still there even a year after the THC user stopped use.


Puberty (ages 13-18) is a critical time of brain development, with neuronal maturation, myelination, synaptic pruning, and dendritic plasticity making this a time of vulnerability to toxic insults.

Other studies have shown structural brain differences with cannabis use.

This study conclusively puts to rest the notion that cannabis is harmless.  You are dumbing down, and causing permanent brain damage, especially if you started your use as a teenager.



Dr. Paul





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