Background Structural interventions focused on community mobilisation to engender an enabling social context have reduced sexual risk behaviours among sex workers. Interventions to date have increased social participation and shown an association between participation and safer sex. Social participation could modify risk for other health behaviours, particularly drug use. We assessed social participation and drug use before and after implementation of a clinical, social and structural intervention with sex workers intended to prevent sexually transmitted infections/HIV infection.

Methods We followed 420 sex workers participating in the Encontros intervention in Corumbá, Brazil, between 2003 and 2005. We estimated the association of participation in external social groups with drug use at baseline and follow-up using logistic regression and marginal modelling. Follow-up analyses of preintervention/postintervention change in drug use employed inverse probability weighting to account for censoring and were stratified by exposure to the intervention.

Results Social participation showed a protective association with drug use at baseline (1 SD higher level of social participation associated with 3.8% lower prevalence of drug use, 95% CI −0.1 to 8.3). Among individuals exposed to Encontros, higher social participation was associated with an 8.6% lower level of drug use (95% CI 0.1 to 23.3). No significant association was found among the unexposed.

Conclusions A structural intervention that modified sex workers’ social environment, specifically participation in external social groups, was associated with reduced drug use. These findings suggest that sexual risk prevention initiatives that enhance social integration among marginalised populations can produce broad health impacts, including reductions in drug use.