• Cocaine;
  • heroin;
  • peers;
  • social influence;
  • social networks;
  • social selection;
  • structural equation modeling


Aims  While several studies have documented a relationship between initiation of drug use and social network drug use in youth, the direction of this association is not well understood, particularly among adults or for stages of drug involvement beyond initiation. The present study sought to examine two competing theories (social selection and social influence) in the longitudinal relationship between drug use (heroin and/or cocaine) and social network drug use among drug-experienced adults.

Design  Three waves of data came from a cohort of 1108 adults reporting a life-time history of heroin and/or cocaine use.

Setting  Low-income neighborhoods with high rates of drug use in Baltimore, Maryland.

Participants  Participants had weekly contact with drug users and were 18 years of age or older.

Measurements  Drug use data were self-report. Network drug use was assessed through a social network inventory. Close friends were individuals whom the participant reported seeing daily or rated as having the highest level of trust.

Findings  Structural equation modeling indicated significant bidirectional influences. The majority of change in network drug use over time was due to change in the composition of the network rather than change in friends' behavior. Drug use by close peers did not influence participant drug use beyond the total network.

Conclusions  There is evidence of both social selection and social influence processes in the association between drug use and network drug use among drug-experienced adults.