An increased risk of psychosis, changes in the brain's reward system and the scrambled neuron signals that may underlie "the munchies" are just some of the many potential effects of marijuana [4] use on the brain.
Links
[1] https://www.livescience.com/55258-how-marijuana-affects-the-brain.html
[2] http://www.rethinkpot.org/tags/brain
[3] http://www.rethinkpot.org/tags/live-science
[4] https://www.livescience.com/46114-marijuana-health-effects.html