We have all heard how marijuana can distort the sense of time, cause short-term memory loss, and increase anxiety and depression.
According to Dr. Gary Emmett, marijuana "may permanently damage a teen's developing brain - and could lead to a reduction in IQ, other drug use and mental health issues."
This supports a longitudinal study conducted in New Zealand that followed 1,037 children born in Dunedin, NZ, in 1972 and 1973. An article on that research published by Time suggested the heaviest users could lose 8 IQ points.
The study's lead author, Madeline Meier of Duke University, explained that "the 8-point decline in IQ for someone with average intelligence (an IQ score of 100, the 50th percentile) would move that person down to the 29th percentile". Meier commented that "the effect of cannabis on IQ is really confined to adolescent users" because the adolescent brain is still developing. The introduction of cannabis might interrupt critical developmental processes.