A recent study shows that those who smoked marijuana at least four times a week and used marijuana throughout their life saw their IQ drop an average of 8 points, the equivalent of going from an A to a B student. The drop was not explained by other drug use, years of education, schizophrenia or using marijuana in the day before the test.
“The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” study researcher Madeline Meier of Duke University said in a statement.
NBC News reported that the study followed 1,037 New Zealand children for 25 years. Subjects took IQ Tests at age 13, before any of them had smoked marijuana, and again at age 38. Throughout the study, participants also answered several surveys about their drug use.
Interestingly, people who picked up the habit as adults had no IQ drop, suggesting that marijuana may not be as harmful to the mature brain.
The findings are the first to associate intelligence declines with marijuana use. Past work linked low IQ and marijuana, but couldn’t rule out the possibility that people who choose to smoke pot are inherently less smart than abstainers.
It’s not clear why marijuana is only bad for teen brain.
One possibility is that teenagers are more vulnerable to marijuana’s effects on brain chemistry, said Susan Tapert, a neuropsychologist at the University of California, San Diego, who was not involved in the study.