Background/Introduction In this study, we investigated intellectual disability support professionals’ knowledge and expectations towards effects of psychotropic drug use on behaviour and drug use in their clients, because shortcomings may lead to misinterpretations of behavioural symptoms and inappropriate drug use. Methods Two self‐designed questionnaires were used to measure the knowledge and expectations of 194 support professionals in 14 residential facilities regarding psychotropic drug use and effects of antipsychotics on behavioural, cognitive and mental functioning of people with intellectual disability. The psychometric properties of both questionnaires were adequate. Results A majority of the professionals had unrealistic expectations regarding the positive effects of antipsychotics on cognitive and behavioural functioning, and 94% scored below the cut‐off scores regarding knowledge; 60% indicated they needed education and training. Conclusions To achieve sufficient collaboration of intellectual disability support professionals in reducing inappropriate psychotropic drug use of clients, vocational educational training is needed.