A four-decade longitudinal study with more than 1,000 participants examined the link between IQ (intelligence quotient) and marijuana use. Subjects had their IQs tested at 13 and again at 38. They were questioned about their marijuana use throughout their lives. Regular marijuana users saw a significant IQ loss between the teens years and adulthood.

Study Results

This study of New Zealand residents found that those who began smoking marijuana during adolescence had significantly lower IQs compared to subjects who either didn’t smoke marijuana or did  not begin smoking until adulthood. Those who began smoking marijuana as teenagers saw an average eight-point drop in their IQs when tested later in life. Those who started using as adults did not see this decrease, and those who never used saw slight increases in IQ. An IQ loss of eight points may not seem like a lot, but it can move a child of average intelligence to the level of one who might need extra help in the classroom.

Research In Action

The period of time between the ages of 12 and 20 is critical for brain development. During this time, myelin coats many of the fibers and allows information to travel more quickly. Using substances during any time is not a good idea, but during adolescence, it can have very long term negative effects, as this research shows.

The research was studying youths who used marijuana in the 1980s. THC (tetrahydrocannabinol, the psychoactive ingredient in marijuana) levels were around 4-5% during this time. Today’s marijuana has levels around 16-18%. This may mean that these findings are actually more conservative than would be shown with the marijuana being smoked today.

ASAP provides treatment for teens who are using substances regularly and education for teens who have used a few times. Call to set up an assessment if you suspect your child has an issue.