BACKGROUND: Increasing trends in the nonmedical use of prescription drugs (NMPD) have been documented, yet little is known about the demographic, severity, and social network influences related to NMPD among treated and untreated problem and dependent drinkers. METHODS: To determine NMPD over four post-baseline interviews, the current study uses a 7-year longitudinal sample of 1598 dependent and problem drinkers, many also using illegal drugs, who were identified through a probability survey in the general population and consecutive intakes in public and private chemical dependency treatment programs in a California county. A panel model incorporating variables from the four post-baseline interviews was used, with baseline variables added for control. RESULTS: The use of illegal drugs (OR=2.18, CI 1.28-3.70), including marijuana (OR=2.66, CI 1.74-4.08), and the nonmedical use of prescription drugs prior to the baseline interview (OR=2.53, CI 1.48-4.33) were related to NMPD over the study period. Larger heavy alcohol and drug using social networks were also related to NMPD (OR=1.03, CI 1.01-1.05). CONCLUSIONS: When examining a sample of dependent and problem drinkers, many of the results obtained were similar to results in the literature predicting drug, and even alcohol, use more generally. In this sample, illegal drug use, and marijuana use were important predictors of NMPD. Thus, the nonmedical use of prescription drugs may not be the first problem noticed by health and psychiatric providers.