A new study suggests that teenagers who smoke pot regularly might be permanently burning a few points off of their IQ.
As the Associated Press explains, participants were tested for IQ twice in the study: once at age 13, and once at age 38. Based on a series of interviews conducted with the participants over time, researchers found that those who used marijuana regularly before the age of 18 showed a drop in IQ score. The same drop in IQ wasn't found for those who began to smoke up regularly after 18.
Essentially, the more participants smoked before age 18, the heavier the IQ loss recorded by the researchers. The study made a demarcation between those who lit up persistently and those who smoked only occasionally. Among participants who used the substance more than 4 times a week under the age of 18, and carried on with the habit for years, the average drop was eight IQ points. The study accounted for abuse of other substances and education disparity. A reduction in use didn't restore the IQ point loss.
The BBC notes that while the study's findings seem to give some weight to the "stoner" stereotype of longtime heavy cannabis users, other factors, such as depression, could also explain both marijuana use and a lower IQ in participants.
The participants were comprised of everyone born in the same New Zealand town over a one-year span, about 1,000 people. Ninety-six percent of original participants saw the study through to completion.