Male substance abuse patients (N=180, 82% African American, mostly low income) who participated in self-help groups (Alcoholics Anonymous, Narcotics Anonymous) after treatment significantly reduced their frequency of alcohol and cocaine use by the 7-month followup.
Participants with high self-help attendance rates used alcohol and/or cocaine less than half as much as did those with low self-help attendance.
This was true regardless of previous substance use and whether or not they completed a 4-week hospital rehabilitation program.
Hence, the effects of self-help groups were not simply due to motivation or other characteristics of the individuals who participated.
McKay, J. R., A. I. Alterman, et al. (1994). Treatment Goals, Continuity of Care, and Outcome in a Day Hospital Substance Abuse Rehabilitation Program. American Journal of Psychiatry 151(2): 254-259.