Estimated rates of co-occurring mental illness among adolescents with substance use disorders range from 60 to 75 percent. Among adolescents with no prior substance use, the rates of first-time use of drugs and alcohol in the previous year are higher in those who have had a major depressive episode than in those who did not. Other commonly documented co-occurring mental disorders include conduct disorder, oppositional defiant disorder, attention-deficit/hyperactivity disorder, anxiety, and post-traumatic stress disorder.

• Youth who experience a major depressive episode were twice as likely to begin using alcohol or an illicit drug, compared to youth who had not experienced a major depressive episode.
• Youth who experienced serious depression were twice as likely to use alcohol as their peers who had not been seriously depressed. Over 29 percent of youth who had not used alcohol previously initiated alcohol use following a major depressive episode within the past year, compared with 14.5 percent of youth who had not experienced a major depressive episode in the past year.
• Similarly, many more youth who had not previously used illicit drugs did so after a major depressive episode. Sixteen percent of youth who had not used an illicit drug in the past year initiated illicit drug use after a major depressive episode, compared with 6.9 percent of youth who had not experienced a major depressive episode in the past year.