the degree of perceived social support from parents and friends (at http://www.emcdda.europa.eu/html.cfm/index3238EN.html) ;
the perception of risk ( at http://www.emcdda.europa.eu/html.cfm/index3461EN.html);
Chapters 2 and 7 concerning theory-driven evaluation (author Liliana Leone),Guidelines for the evaluation of drug prevention - Secomd Edition EMCDDA, Lisbon http://www.emcdda.europa.eu/attachements.cfm/att_184810_EN_188138_EMCDDA_Guidelines.pdf. Prevention and Evaluation Resources Kit (PERK) vol 4
I would say there are two main components- Peer related expectations and drug effect expectations for the user.
Peer related are driven by the usual social factors. Drug Effect expectations are driven by anecdotal accounts from people whom the user likely trusts.