ObjectivesTo describe the use, characteristics and influence of lay consultants on treatment-seeking decisions of adults in slums of Nigeria.DesignCross-sectional survey using a pre-piloted questionnaire.SettingsTwo slum communities in Ibadan city, Nigeria.Participants480 adults within the working age group (18–64).ResultsMost respondents (400/480, 83.7%) spoke to at least one lay consultant during their last illness/health concern. In total, 683 lay consultants were contacted; all from personal networks such as family and friends. No respondent listed online network members or platforms. About nine in 10 persons spoke to a lay consultant about an illness/health concern without intending to seek any particular support. However, almost all (680/683, 97%) lay consultants who were contacted provided some form of support. Marital status (OR=1.92, 95% CI: 1.10 to 3.33) and perceiving that an illness or health concern had some effects on their daily activities (OR=3.25, 95% CI: 1.94 to 5.46) had a significant independent association with speaking to at least one lay consultant. Age had a significant independent association with having lay consultation networks comprising non-family members only (OR=0.95, 95% CI: 0.92 to 0.99) or mixed networks (family and non-family members) (OR=0.97, 95% CI: 0.95 to 0.99), rather than family-only networks. Network characteristics influenced individual treatment decisions as participants who contacted networks comprising non-family members only (OR=0.23, 95% CI: 0.08 to 0.67) and dispersed networks (combination of household, neighbourhood and distant network members) (OR=2.04, 95% CI: 1.02 to 4.09) were significantly more likely to use informal than formal healthcare, while controlling for individual characteristics.ConclusionsHealth programmes in urban slums should consider engaging community members so, when consulted within their networks, they are able to deliver reliable information about health and treatment-seeking.