Marijuana is addictive. More teens are in treatment with a primary diagnosis of marijuana dependence than for all other illicit drugs combined.
Source: Treatment Episodes Data Set (TEDS) 1992-2002, SAMHSA, 2006
Young people who use marijuana weekly have double the risk of depression later in life.
Source: Patton, GC et al. "Cannabis use and mental health in young people: cohort study." British Medical Journal, 325: 1195–1198, 2002
Heavy Marijuana users are more likely than non-users to be diagnosed with schizophrenia later in life. A recent study found that people who had used marijuana more than 50 times before the age of 18, had a three fold increased risk of developing schizophrenia later in life.
Source: Zammit, S et al., "Self-reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study," British Medical Journal, 325:1199-1201, 2002 Arseneault L, et al., "Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study," British Medical Journal, 325:1212-1213, 2002
Marijuana can cause increased heart rate and make some users extremely anxious or paranoid.
Source: NIDA. Marijuana: Facts for Teens, 2008
Heavy marijuana use impairs young people's ability to concentrate and retain information.
Source: Pope HG et al., "Early-onset cannabis use and cognitive deficits: what is the nature of the association?" Drug and Alcohol Dependence, 69(3): 303-310, 2003.
The short-term effects of marijuana can include problems with memory and learning.
Source: NIDA. Marijuana: Facts for Teens, 2008
