Misleading and scientifically illiterate claims notwithstanding, there's little definitive evidence marijuana makes you dumb, dense, or crazy.

Scare-mongering headlines touting the alleged adverse effects of marijuana’s impact on brain appear in the mainstream media almost daily. But a careful investigation of the relevant science reveals many of these fears to be overblown at best and illegitimate at worst.

Pot Permanently Damages the Brain

“Even occasional marijuana can change your brain.” So claimed the headlinessurrounding a highly publicized 2014 study by researchers at Northwestern University’s Feinberg School of Medicine and the Massachusetts General Hospital Center for Addiction Medicine. Authors took images of the brains of college-aged cannabis users and non-users and acknowledged differences in the brain morphology of the two groups.

However, because the study was based on a single set of images, researchers had no way to determine whether the differences predated subjects cannabis use or whether they would remain...