Heavy marijuana use at a young age can significantly lower a person's IQ.

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Becoming a heavy cannabis smoker as a teenager results in cognitive decline not seen if the illicit drug use starts when adult.

Clinical psychologist Madeline Meier at Duke University in Durham, North Carolina, and her colleagues used data from the famous Dunedin Longitudinal Study, an ongoing multi-factor survey involving a cohort of 1,037 New Zealanders followed from birth, which now has 40 years worth of data. Participants in the Dunedin Study had been periodically tested for IQ and other neuropsychological indices as well as being asked about behaviour such as drug-taking. The study was published in the Proceedings of the National Academy of Sciences on Monday1.

When their adult IQ was tested at 38 years old, the heaviest and most persistent adolescent-onset users in the study had experienced an average decline of eight IQ points from childhood to adulthood. Non-users had on average increased their IQ by around one point. And even after setting aside the heaviest users, a decline of a few IQ points from their childhood value was still seen in  less heavy users who had started in their teens. What's more, the drop in mental function seemed irreversible even after people had quit cannabis. However, persistent users who only started when adult (older than 18 years) did not seem to experience the same IQ decline.

 

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