I quickly found an article on WebMD that refers to a study showing early, long-term marijuana use is linked to a drop in IQ.
Source
“The research included slightly more than 1,000 adult participants in a New Zealand health study who were followed from birth.
As part of the study, IQ tests were performed at age 7, 9, 11, 13, and 38.
For the latest analysis, Duke University postdoctoral researcher Madeline H. Meier, PhD, and colleagues compared IQ scores at age 13—before most of the participants had used marijuana—to those at age 38, when many had experience with the drug.
In addition to the IQ tests, family members or close friends completed questionnaires when the participants were 38 years old designed to determine if the study enrollees had trouble with attention, memory, or social functioning.”
“People who began using marijuana in their teens and continued to use the drug for many years had an average decline in IQ of eight points by the time they reached 38 years old.
The decline could not be explained by the use of alcohol or other drugs or by having less education, and the effect was not fully reversed in those who stopped using marijuana.”
Read the rest to get all the details.