The downside to cannabis use has been made clearer. The most detailed study yet of the drug's long-term effects shows that those who start a weed habit as teens enter middle age with an 8-point deficit in IQ compared to non-users.
Evidence is growing that cannabis-based drugs can benefit health, but suspicions remain that persistent cannabis use from an early age can have a detrimental effect on cognition. Confirming those suspicions is tricky, though, since cognitive impairment observed later in life could have been present before the drug was first used.
To get around the problem Madeline Meier at Duke University in Durham, North Carolina, and her colleagues have taken the long view. They used a detailed health study which followed 1000 people in Dunedin, New Zealand, from birth until age 38. The data allowed them to compare IQ tests taken by the participants at age 13 ? before any of them began using cannabis ? with the same participants' IQ scores as adults, in some cases after years of cannabis use.
The study showed that those who developed the most persistent dependence on the drug showed the greatest subsequent decline in IQ, losing 6 points on average regardless of how early the habit began. Within that group, those who began taking the drug before their 18th birthday saw a subsequent decline in IQ of 8 points, on average.
Furthermore, friends and relatives close to the persistent cannabis users reported that these users had more everyday memory and attention problems, including forgetting to pay bills and misplacing common items like keys and wallets.
The bad news is that the damage does not appear to be reversed after dropping the habit. But the good news is that people who picked up their drug habit after their 18th birthday did not suffer such severe cognitive decline. Although previous research has hinted at the potential impacts of cannabis on the adolescent brain, this study is the first to provide evidence that cannabis does in fact have neurotoxic effects on young brains, says Meier.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1206820109
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