Background: Polypharmacy and chronic drug use are common in people with intellectual disability and behavioural problems, although evidence of effectiveness and safety in this population is lacking. This study examined the effects of a structured medication review and aimed to improve pharmacotherapy in inpatients with intellectual disability. Methods: In a treatment facility for people with mild to borderline intellectual disability and severe behavioural problems, a structured medication review was performed. Prevalence and type of drug-related problems (DRPs) and of the recommended and executed actions were calculated. Results: In a total of 55 patients with intellectual disability and behavioural problems, 284 medications were prescribed, in which a DRP was seen in 106 (34%). No indication/unclear indication was the most prevalent DRP (70). Almost 60% of the recommended actions were also executed. Conclusions: This high prevalence of DRPs is worrying. The structured medication review is a valuable instrument to optimize pharmacotherapy and to support psychiatrists in adequate prescribing of both psychotropic and somatic drugs.