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Additional file 1: Supplementary Methods. Supplementary Results. Table S1. Weighted multivariable logistic regression models of multiple sexual partners, casual sex, group sex, and rare condom use on early sexual initiation among alcohol- and tobacco-using young adults in Taipei metropolitan area recruited using RDS during 2007–2010 (N = 916). Table S2. Condom use (in original options) among sexually experienced alcohol- and tobacco-using young adults in Taiwan recruited using RDS during 2007–2010, by gender and sexual initiation (N = 916). Table S3. Relations of sexual partners and relationship status to condom use (in original options) among sexually experienced alcohol- and tobacco-using young adults in Taiwan recruited using RDS during 2007–2010, by gender (N = 916). Table S4. Relations of sexual partners and relationship status to condom use (in original options) among sexually experienced alcohol- and tobacco-using young adults in Taiwan recruited using RDS during 2007–2010, by gender and early sexual initiation (N = 916). Table S5. Weighted multivariable logistic regression analysis of rare condom use on early sexual initiation among alcohol- and tobacco-using young adults in Taiwan recruited using RDS during 2007–2010 (N = 916).

Additional file 2. Questionnaire for the variables used in the study.

Additional file 1: Table S1. The proportion of lifetime ketamine use and male gender in each year of the RDS-sample in Taipei Metropolitan Area. Table S2. Univariate logistic regression analysis of any ketamine use on individual ketamine expectancies (unweighted) among 162 ever users of ketamine and 840 illicit drug-naive. Table S3. Multivariable logistic regression analysis of any ketamine use on ketamine expectancies (unweighted) among 162 ever users of ketamine and 840 illicit drug-naïve. Table S4. Ketamine-using history of the RDS-sample in Taipei Metropolitan Area (N = 1115), by illicit drug use experience. Table S5. Distribution of ketamine expectancies among early initiation of tobacco and alcohol, respectively. Table S6. Multinomial logistic regression model of illicit drug use experience (reference group: illicit drug-naïve) on continuous ketamine expectancies with adjustment for sociodemographics and early-onset tobacco use (N = 1115). Table S7. Multinomial logistic regression model of illicit drug use experience (reference group: illicit drug-naïve) on binary ketamine expectancies with adjustment for sociodemographics and early-onset alcohol use (N = 1112). Table S8. Multinomial logistic regression model of illicit drug use experience (reference group: illicit drug-naïve) on binary ketamine expectancies with interaction terms involving early-onset tobacco use (N = 1115). Table S9. Multinomial logistic regression model of illicit drug use experience (reference group: illicit drug-naïve) on binary ketamine expectancies with interaction terms involving early-onset alcohol use (N = 1112)