Teen Use of Marijuana Can Lower IQ: Studies Show Pot Damage Permanent

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Marijuana is not good for developing teen brains. Image courtesy of the United States Fish and Wildlife Service

As increasing numbers of states move to legalize marijuana use, it is important for both adolescents and adults to understand how exposure to cannabis use affects the developing teen brain.

Currently six to seven percent of high school seniors regularly use marijuana. However, studies link regular marijuana exposure during the teen years to lowered IQ, higher dropout rates, and further drug use.

Adolescent brains exposed to marijuana lose neural connectivity as they develop, whereas adult brains have finished developing neural pathways and do not show the same loss in neural function from marijuana use.

Does Heavier Marijuana Use Lower IQ?

The Dunedin Study in New Zealand followed over 1000 individuals from birth to age 38. Individuals were tested for IQ and brain function at ages 13, and again at 38.

Over the course of the study the individuals were interviewed about their marijuana use. Those who consistently used marijuana showed a decline in regular brain functions.

The most significant declines were noted in individuals who began their marijuana use as adolescents. Those who started after age 18 were found to have only minor declines in brain function.

Participants with no reported use showed a slight increase in IQ between ages 13 and 38. Those who showed dependence on marijuana lost an average of up to 6 IQ points.

Pot and Mental Functions: Questions and Answers

Which mental functions were affected?

The researchers used the Wechsler Adult Intelligence scale that measures four mental functions. According to the study, all functions, working memory, processing speed, perceptual reasoning and verbal comprehension were affected by pot use.

Could something else be responsible for the difference in IQ, not marijuana use?

When several other explanations were eliminated from the study, the results remained the same. The authors ruled out hard drug dependence, alcohol dependence, schizophrenia, and recent marijuana use (both past 24 hour and past week use).  It is possible something else is responsible for the difference, but not likely.  The most likely explanations were accounted for in the study.

What about education? Some people received more than others.

The mental loss from marijuana remained significant, even among just those users with a high school education or less. It was noted, however, that fewer regular marijuana users received education after high school.  A difference in mental ability from marijuana use was noted in each education group, not just from one group to another.

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