Background: Antipsychotic drugs are used in the routine treatment of adults with intellectual disabilities (ID) and challenging behaviour in the UK despite limited evidence of their effectiveness. There is no evidence on their cost-effectiveness. Methods: The relative cost-effectiveness of risperidone, haloperidol and placebo in treating individuals with an ID and challenging behaviour was compared from a societal perspective in a 26-week, double-blind, randomised controlled trial. Outcomes were changes in aggression and quality of life. Costs measured all service impacts and unpaid caregiver inputs. Results: After 26 weeks, patients randomised to placebo had lower costs compared with those in the risperidone and haloperidol treatment groups. Aggression was highest for patients treated with risperidone and lowest for patients treated with haloperidol; however, quality of life was lowest for patients treated with haloperidol and highest for patients treated with risperidone. Conclusion: The treatment of challenging behaviour in ID with antipsychotic drugs is not a cost-effective option.