Abstract Aims Severe behavioural problems (SBP) are a major contributor to morbidity in children with intellectual disability (ID). Medications used to treat SBP in ID are associated with a high risk of side effects. Cannabidiol has potential therapeutic effects in SBP. This pilot study aimed to investigate the feasibility of conducting a randomised placebo-controlled trial of cannabidiol to reduce SBP in children with ID. Methods This is a double-blind, placebo-controlled, two-armed, parallel-design, randomised controlled trial of cannabidiol in children aged 8–16 years with ID and SBP. Participants were randomised 1:1 to receive either 98% cannabidiol in oil (Tilray, Canada) or placebo orally for 8...