Marijuana use is clearly associated with poorer memory, learning, decision making, impulsivity and attention. Some have wondered whether these problems are caused by marijuana or predated marijuana use. A recent review (Crean, Journal of Addictive Medicine 5:1-8, 2011) concluded that marijuana use itself can cause these. It also examined whether these problems persisted after cessation and concluded that most all eventually went away, although sometimes it took several weeks. The exception was poor impulse control. Thus, it may be that more impulsive people start marijuana and marijuana actually makes them even more impulsive.
–Dr. John R. Hughes