No, marijuana use doesn’t lower your IQ

A 2012 Duke University study made international headlines when it purported to find a link between heavy marijuana use and IQ decline among teenagers. Other researchers questioned the findings almost immediately: Columbia University's Carl Hart noted the very small sample of heavy users (38) in the study, leading him to question how generalizable the results were.

Then, a follow-up study published 6 months later in the same journal found that the Duke paper failed to account for a number of confounding factors: "Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature," it concluded.