Interactive effects of drug use and social vulnerability on health outcomes among street involved youth in a prospective cohort study from Indonesia

Background/objectives Street-involved youth experience overlapping behavioral and structural vulnerabilities that increase the risk of adverse health outcomes. However, longitudinal evidence examining whether social vulnerability modifies the health effects of drug use remains limited, particularly in low- and middle-income countries. This study investigated the independent and interactive effects of drug use and social vulnerability on adverse health outcomes among street-involved youth in urban Indonesia. Methods A prospective cohort study included 364 street-involved youth aged 15–24 years in Makassar City, South Sulawesi, Indonesia. Purposive sampling was used, with recruitment through the Drug Abuse Prevention Programme and Mitra Husada Foundation support network. Eligible youth were identified, approached, and screened through these networks by four facilitators. Consenting participants were enrolled at baseline and followed for 12 months. Drug use was assessed at baseline, and social vulnerability was measured using housing instability, income insecurity, and duration of street involvement. Multivariable Poisson regression with robust variance estimated adjusted relative risks (RRs) and 95% confidence intervals (CIs). Results Drug use (RR = 1.76; 95% CI: 1.32–2.34) and high social vulnerability (RR = 1.48; 95% CI: 1.12–1.96) were independently associated with increased risk of adverse health outcomes. A significant interaction effect was observed (RR = 1.35; 95% CI: 1.02–1.79), indicating that combined exposure amplified health risks. Participants exposed to both drug use and high social vulnerability experienced the greatest risk (RR = 1.89; 95% CI: 1.41–2.52). Conclusions Drug use and social vulnerability jointly contribute to adverse health outcomes among street-involved youth. Integrated and equity-oriented interventions addressing both behavioral and structural determinants of health are needed to reduce health inequalities and improve health outcomes among vulnerable urban youth populations.