This article describes obstacles to the rational use of psychiatric drugs in a 1000-bed institution for people with intellectual disability employing the WHO framework for rationality. Quantitative data were collected from 98 charts and qualitative data from 14 individuals in this case study. Eight-three percent of patients were taking psychiatric medications, even though 67% of these had no psychiatric diagnosis. Antipsychotics were the first-order treatment; antidepressants were rarely prescribed. These prescribing patterns are influenced by institutional culture, including an emic construct of retardation as a form of psychosis. Low staff-to-patient ratios, a lack of non-medical treatments, and the hospitalization of patients for social reasons also contribute to medication misuse.