The persistent use of cannabis before age 18 has been linked to lasting harm to intelligence, according to a large study.
Analysis of more than 1000 New Zealanders found those who took up cannabis in adolescence and used it for years afterwards experienced an average decline in IQ of eight points when measured at age 13 and 38.
People who did not begin using cannabis until they were adults, with fully formed brains, did not show the same declines.
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