Background: 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. Aims: To investigate the factors, and their relative importance, that influence the decision to initiate medication in adults, and parents of children, with ADHD. Methods: 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. Results: 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. Conclusions: 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.