New Study Shows Long-Term Drop in IQ for Teens Who Frequently Smoke Marijuana

A new study published online in the Proceedings of the National Academy of Sciences, conducted by researchers at Duke University's Center for Child and Family Policy, shows that teens who frequently smoke marijuana are more likely to suffer a long-term drop in IQ. This is a huge potential concern, as recent numbers show that approximately one in 10 teenagers, 9th through 12th grade, smoke marijuana at least 20 times a month.
 
The study involved more than 1,000 participants, all born in Dunedin, New Zealand in 1972-73, who were IQ tested at age 13, before any substantial marijuana use, and again at age 38. They were also interviewed 5 times about their marijuana use, between ages 18 and 38. IQ scores at age 13 were compared to those at age 38, showing an IQ drop on average of 8 points only in those who had started smoking marijuana on a regular basis by 18 years old, even if they almost or totally quit smoking pot at age 38.
 
A co-author of the study, Dr. Richie Poulton, Professor of Developmental Psychopathology at the University of Otago in New Zealand, said the research points to the importance of avoiding marijuana until adulthood if possible. Dr. Staci Gruber of Harvard-affiliated MacLean Hospital’s Brain Imaging Center, who wasn’t involved in the study, said the suggestion that marijuana hurts the adolescent brain is “something we believe is very likely,” and this recent finding of a drop in IQ deserves further study.