Study: Cannabis IQ Drop is a Myth

December 30, 2019

Does weed make you dumb?

The idea that regular cannabis use can leave you with lower intelligence is fairly common—and can even be supported by some scientific research. But a new twin study is coming to cannabis’ defense, debunking this old idea with stronger evidence.

Early research points to cannabis lowering IQ

This notion that cannabis causes lower cognitive functioning isn’t completely unsupported. While the idea has been floating around in anti-cannabis propaganda for decades, its scientific basis can largely be traced back to a 2012 longitudinal study that found that cannabis use in adolescence was tied to lower IQ and cognitive functioning scores in young adulthood. The study followed participants from age 13 to 38, interviewing and testing them to assess IQ, executive functioning, and cannabis use patterns. Those who started using cannabis during adolescence saw drops in their cognitive scores later on. And of these subjects, those who used the most cannabis use saw worse drops in IQ.

Unsurprisingly, researchers concluded from this that cannabis must have neurotoxic effects on the adolescent brain, causing cognitive decline if used at a young age. Other studies began to replicate the result, and the scientific community started repeating the story. Quickly the standard line had become “cannabis use causes cognitive decline—especially for teens.”

But as it turns out, that conclusion suffers from a really common error: correlation vs. causation.

Twin study debunks 2012 research

The evidence might seem strong that using cannabis leads to cognitive decline, but a recent 2019 twin study is questioning the methodology that led to the 2012 results, suggesting that we’ve been making unfounded assumptions about cannabis’ role in IQ drop.

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