“What’s clear, however, is that marijuana’s signature high comes from a psychoactive component known as tetrahydrocannabinol (THC). And evidence is mounting, says Weiss, that THC is not risk-free.
In the short term, marijuana use has been shown to impair functions such as attention, memory, learning and decision-making. Those effects can last for days after the high wears off. Heavy marijuana use in adolescence or early adulthood has been associated with a dismal set of life outcomes including poor school performance, higher dropout rates, increased welfare dependence, greater unemployment and lower life satisfaction. ”
http://www.apa.org/monitor/2015/11/marijuana-brain.aspx