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Teenagers who smoke marijuana regularly may be doing permanent brain damage, reducing IQ by up to eight points, according to a new study.

Researchers compared Intelligence Quotient (IQ) results for subjects aged 13 and then at 38 for more than 1,000 New Zealanders, some of whom were regular cannabis-users and some who were not.

The results, published in the Proceedings of the National Academy of Sciences, were striking.