The decision to initiate medication is complex and is influenced by a variety of factors. There is limited information on the relative importance of factors that influence the initiation of ADHD medication.To investigate the factors, and their relative importance, that influence the decision to initiate medication in adults, and parents of children, with ADHD.A discrete choice experiment was conducted using eight choice tasks made up of five attributes that described the outcomes of initiating medication. A mixed multinomial logit model was used to estimate preferences for medication.Respondents' overall preferences for initiating medication were negative (Mean (β)= -0.72705, p < 0.1), however, significant heterogeneity was noted in preferences (SD: 0.93604, p < 0.001). Side-effects were the most important factor for both adults (Relative importance (RI) = 40.39%) and parents (RI = 41.99%). Improvement in education had a greater weighting in adults' decision-making compared to parents (RI = 36.93% vs 30.47%) while improvement in aggressive (RI = 14.38% vs 11.84%) and social behaviour (RI = 12.59% vs 10.37%) was more important to parents.Important differences in preferences of patients and parents were identified, highlighting that the decision to initiate medication is influenced differently in different individuals and groups.