BACKGROUND:We report prevalence and incidence of drug use initiation in Australian gay and bisexual men (GBM) participating in an online cohort study. METHODS:Between September 2014 and June 2015, 1,710 GBM were enrolled in the Following Lives Undergoing Change Study and followed-up six monthly. Participants were asked about measures of lifetime use at baseline and recent use (last six months) at all visits. Drug use initiation was defined as men who reported having never used a specific drug prior to baseline and reported recent use at follow-up. RESULTS:Participants' median age was 31 years (range: 16-81). Prevalence of lifetime use was significantly associated with older age for all individual drugs (p trend<0.001), and 84.1% reported lifetime use of any drugs. Just above half (51.9%) reported recent use at baseline, with the majority reporting occasional use (once or twice in the previous six-months). Among men who reported no history of drug use at baseline, drug initiation was highest for amyl nitrite, with an incidence of 10.5 per 100 person-years (95% CI 7.9-13.9), followed by cannabis (7.3 per 100 person-years, 95% CI 5.0-10.6) and ecstasy (5.0 per 100 person-years, 95% CI 3.6-7.0). Younger age was significantly associated with higher incidence of initiation of amyl nitrite, ecstasy, cocaine, ketamine, GHB, and LSD (p trend <0.05 for all). CONCLUSION:Prevalence of lifetime illicit drug use is lower in younger GBM than in their older counterparts. However, incidence of drug use initiation is high among younger men, providing an opportunity for early intervention.