Background and Objective: Poor healthcare in urban slums in developing regions represents a complex humanitarian crisis requiring innovative and bottom-up solutions. How slum dwellers manage their health despite huge health risks and poor healthcare access, by utilising their social connections and social capital, is poorly understood. Drawing on social network theory, this study examined how slum dwellers in Nigeria used their Lay Consultation Networks (informal and naturally occurring networks with whom people discuss illness symptoms and health problems) to make treatment decisions. Method: A mixed-method approach was adopted. Surveys (n=480) and semi-structured interviews (n=30) were conducted among working-age adults in two slums in Ibadan (large south-western city), Oyo state, Nigeria. A public reference group comprising community members and researchers at the University of Ibadan was constituted to guide the study. Results: Most participants consulted 1 to 3 informal network members during illness. Those consulting non-family members were significantly more likely to use alternative medicine than formal care. Use of online-based networks was scarce mainly due to poor digital literacy, and lack of digital devices. Network members shared advice about prescriptions and treatments (learnt from others or through their own experiences) with ill persons, some useful and some not. Limited neighborhood trust, busy work schedules, discrimination, and stigmatization of certain groups contributed to shaping who participants could talk to. Participants displayed a strong sense of agency when implementing advice or support from others. Conclusions: Lay consultation networks can be an integral part of the overall health system. They can be incorporated into health policies and programmes to increase coverage and effectiveness of health interventions in slums and disadvantaged communities in low-and-middle-income countries.