Marijuana Use & its Impact on Intelligence

The National Institute of Health (NIH) is a federal agency dedicated to research in biomedical and public health.  The NIH is composed of 27 separate institutions and centers from various scientific disciplines.  In 2019, the NIH was ranked second in the entire world for biomedical sciences by Nature Index when measured by the largest contributors to papers published in leading journals from the preceding 4 years.  One of the NIH’s institutes in the National Institute on Drug Abuse. 

In August of 2013, the National Institute of Drug Abuse published findings from a 25-year study on marijuana use and cognitive abilities.  What the study found was that regular marijuana use (more than 4 days a week) beginning before age 18 was associated with an average Intelligence Quotient (IQ) decline of 8 points by age 38.  That 8 points of IQ loss amounts to falling from the 50th percentile to the 29th percentile.  In terms of an ACT score, the 50th percentile in science is a 20 while the 29th percentile is just over 15.  

The NIH study found that memory, processing speed, executive functions, verbal skills, and attention (i.e., virtually every kind of brain function) were all measurably reduced.  And the loss in intelligence was not merely detected by the testing instruments, but the loss of cognitive ability in the marijuana smokers was quite apparent to those individuals close to the smoker.  Worse yet, researchers found the impact on cognitive ability was drastically worse on adolescent, developing brains.  

Injustice for All: The (Familiar) Fallacies of Criminal Justice Reform, Brian Surber (True Blue Publishing, 2021) p. 100.