Objective This study examined the relationship between two risk factors for substance misuse (self-control, substance using friends) and changes in jail inmates' substance misuse from pre-incarceration to post-release. Method Participants were 485 adult jail inmates held on a felony conviction, recruited from a metropolitan county-jail situated in the mid-Atlantic region of the U.S. During incarceration, participants completed self-report assessments of pre-incarceration substance misuse and self-control. At one-year post-release, participants reported their substance misuse and proportion of substance-using friends (n = 322 at follow-up). Results The relationship between self-control and changes in inmates' substance misuse was fully mediated by association with substance-using friends. Age moderated the relationship between friends' substance use and changes in inmates' own misuse of marijuana and cocaine. Friends' use was more strongly related to marijuana misuse for younger adults than for older adults. However, for cocaine misuse, this relationship was stronger for older adults than for younger adults. Self-control's relationships with other variables were not moderated by age. Conclusions This study underscores importance of self-control's indirect relationship (through substance-using friends) with changes in substance misuse: inmates with lower self-control were more likely to associate with substance-using friends and, in turn, had more symptoms of substance misuse 1-year post release. Results emphasize the importance of considering adult substance-users' friend-relationships. However, age and type of substance appear important when considering the relative importance of friends' influence.
