Investigators from San Diego State University/University of California looked at whether, for dual diagnosis veterans with substance dependence and major depressive disorder, 12-Step success with alcohol and drug use might be mediated by reductions in depression. 

Veterans (209) with this dual diagnosis (chemical dependency and depression) were enrolled in this controlled trial, randomized to either Twelve-Step Facilitation (TSF) or Integrated Cognitive-Behavioral Therapy (ICBT), delivered in out-patient groups for 6 months, with support from medication.

Twelve-Step attendance and affiliation, depression severity, percentage of days drinking and percentage of days using drugs were assessed at baseline and at months 3, 6 and 9.

Greater 12-Step meeting attendance predicted lower depression and mediated the superior depression outcomes of the TSF group, explaining 24.3% of the group difference in depression.

Independent of treatment group, lower depression severity predicted lower future alcohol use and mediated the effects of 12-Step meetings, explaining 15.7% of their effects on future drinking.
These effects were not confounded by current substance use, suggesting that depression has unique associations with 12-Step meeting attendance and future drinking.

The research team concludes that for patients with substance dependence and major depressive disorder, attendance at 12-Step meetings is associated with mental health benefits that extend beyond substance use.
In fact, reduced depression could be a key mechanism whereby 12-Step meetings reduce future drinking in this population.

Citation:  Worley MJ1, Tate SR, Brown SA. Mediational relations between 12-Step attendance, depression and substance use in patients with comorbid substance dependence and major depression. Addiction. 2012 Nov; 107 (11): pages 1974-83.