Understanding how spatial interactions of built environment features shape substance-use risk among youth in a rapidly urbanizing Nigerian city.

<jats:p>Background: Adolescent and youth substance use is an important public health concern in rapidly urbanizing low- and middle-income countries; however, evidence on how social networks and community substance-use environments jointly shape recent use remains limited, particularly in African megacities. Methods: We conducted a cross-sectional, community-based, convergent mixed-methods study of adolescents and young adults aged 12-24 years in the Yaba Local Council Development Area, Lagos, Nigeria. Quantitative data were collected using a structured questionnaire adapted from established, international survey instruments. The primary outcome was self-reported substance use within the past 30 days. Key exposures included a Social Exposure Score incorporating substance use among friends and family members, membership in a substance-using peer group, and perceived easy community availability of substances. Multivariable logistic regression was used to examine factors associated with past-30-day substance use, followed by an interaction model to assess whether perceived availability modified the association between social exposure and recent use. Open-ended responses on community approaches to reducing substance use were thematically analyzed and integrated with quantitative findings through a joint display. Results: Among 285 participants (mean age 18.9 years; 52.6% male), 66 (23.2%) reported past-30-day substance use and 84 (29.5%) reported lifetime polysubstance use. A Higher Social Exposure Score was associated with increased odds of past-30-day substance use (adjusted odds ratio [aOR]=2.18; 95% CI: 1.59-2.99; p&lt;0.001), while perceived easy community availability was independently associated with recent use (aOR=3.22; 95% CI: 1.42-7.30; p=0.005). The interaction between social exposure and perceived availability was statistically significant (aOR=1.51; 95% CI: 1.01-2.27; p=0.047), indicating that the association between social exposure and recent use varied according to perceived availability. The marginal effect of a one-unit increase in the Social Exposure Score on the predicted probability of past-30-day use was +0.08 when easy availability was not reported and +0.18 when it was reported. Among lifetime substance users, past-30-day use was more common among polysubstance users than single substance users (50.0% vs. 22.0%; chi-square[1]=16.51; p&lt;0.001). Qualitative findings identified supply side law enforcement (43.5%), population awareness campaigns (24.6%), regulatory and legislative control (16.1%), enhanced parental supervision (14.7%), and economic and youth empowerment (13.3%) as prominent community-proposed solutions. Integrated analysis demonstrated complementarity between the quantitatively identified social and environmental correlates and community-proposed intervention priorities. Conclusions: In this urban Nigerian setting, past-30-day substance use was independently associated with social exposure and perceived community availability, with evidence that the association between social exposure and recent use varied according to perceived availability. The findings support multilevel prevention approaches that address environmental access alongside peer, family, community, and broader socioeconomic influences.</jats:p>