The aim of this study was to evaluate the efficacy of a theory-based intervention to increase the use of a new syringe for each injection among injection drug users (IDUs). Users of two needle exchange programs (NEPs) were involved. At both sites, participants were assigned at random to either the experimental or the control group. Once a week for four weeks, users reported to the NEPs where they logged onto a computer and received an audiovisual message. A total of 260 IDUs were recruited. At baseline, 52.3% of participants reported that they had not always used new syringes in the previous week. The results indicate that it is possible for IDUs to adopt safer injection practices. One month after the intervention began, participants in the experimental group were using fewer dirty syringes compared to the control group (RR: 0.47 CI(95%) 0.28-0.79; P = .004). This short-term effect was no longer present 3 months later.