Marijuana causes brain cells to die, concluded a Tulane researcher in 1974. Dr. Robert Heath breathlessly warned media that “we seem to be playing with dangerous, dangerous stuff,” when it comes to weed. Not only were his findings debunked by later studies, it turns out the doctor had pumped marijuana smoke through gas masks attached to the monkeys’ faces. Asphyxiation caused the death of brain cells in the poor monkeys – not cannabis.

Even so, the myth that marijuana makes people dumb continues to endure. These days, anti-cannabis advocates warn about weed’s “likely connection with cognitive decline.”

But a new study in JAMA Psychiatry “found little evidence for more severe effects with cannabis use at earlier ages.” The research, led by University of Pennsylvania researcher Dr. J. Cobb Scott, sought to get to the bottom of the issue with a meta-analysis of 69 studies that examined cannabis use by young people. More than 2,100 marijuana users who averaged about 20 years old were included.

“In light of the changing perceptions of cannabis use and an evolving policy landscape surrounding cannabis, understanding [its] potential risks… is of paramount importance,” wrote the researchers.

The study found that heavy cannabis use is indeed associated with a small reduction in cognitive functioning in young people, but these reductions disappeared once users abstained from marijuana for 72 hours.

While individual studies may offer conflicting conclusions, meta-analyses that look at all the research as a whole are able to draw broader conclusions. Even with the link between cannabis and reduced cognitive functioning, the association “may be of questionable clinical importance for most individuals,” wrote the researchers.

The conclusion “is not surprising,” said Paul Armentano, the Deputy Director of the National Organization to Reform Marijuana Laws. In an email, Armentano pointed out studies on twins that failed to find any cognitive declines due to adolescent marijuana use.

“These findings… dispute the long-standing ‘stoner-stupid’ stereotype,” he said.

Researchers emphasized that additional “large-scale longitudinal studies are needed.” But perhaps prohibitionists should refocus their efforts on the negative effects of alcohol use over cannabis.

Sign up for our newsletter Subscribe Nice email address!