Using cannabis as a teenager can cause a permanent and irreversible drop in IQ, according to a study of 1,000 people in the New Zealand town of Dunedin.

Prof Terrie Moffitt told the Today programme that people using cannabis fall "about 20% behind in the population on where they would have been based on their native intelligence that they had in primary school".

She explained that the findings were based on "heavy use at a young age", meaning people aged under 18 using cannabis on at least four days a week.

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