The widely mocked stoner sloth anti-drug campaign typifies a long-held stereotype about marijuana—it makes you slow, stupid, dull. There’s been some research backing up the stereotype, suggesting that marijuana usage can be linked to declines in intellectual function.

Source: http://arstechnica.com/science/2016/01/marijuana-might-not-be-the-culprit-in-adolescent-iq-decline/