Scientific research seems to support mal-effects of marijuana on human and rat brains especially if regular use starts in youth. Apparently it is bad for pregnant women too, and not just for herself. The progeny may suffer damage.
Cannabinoid receptors in the brain in numerous reagions may be effected by reguar dope use especially in adolescence. Loss of neurons in the hippocampus may be accelerated with the aged hop-head brain. Memory loss and verbal impairment begin starting in adolescence can continue until the doper’s last toke in the old.
http://www.politico.com/states/new-jersey/story/2017/08/01/booker-seeks-federal-marijuana-legalization-113716 Booker wants to legalize dope nationwide
Abornirmalities have been found in the white matter of the corpus callosum that connects the two haves of the brain among dope users of high-THC pot
The corpus collosum is the light area at center in this image of the brain of a formerly healthy dead human https://www.blogger.com/Public%20Domain,%20https://commons.wikimedia.org/w/index.php?curid=295383