The present study examined social control processes in drug cessation among adults. Social control theory posits that the association between drug use and the drug use of a person's social network results from an individual seeking out similar peers. The data included 629 individuals who reported past-year heroin or cocaine use at baseline and had follow-up data in a community study in Baltimore, MD. Negative binomial regression modeling indicated that the reduction in social network drug use was significantly greater for quitters than those who did not quit. Compared to non-quitters at baseline, the incidence rate ratio (IRR) of the number of drug-using network members was 0.86 for quitters at baseline, 0.71 for non-quitters at follow-up, and 0.28 for quitters at follow-up (all p<0.05). These findings support social control theory in adult drug use cessation. Future research should extend the length of follow-up and assess bidirectional influences.