Psychopharmacological and behavioural interventions are used to treat challenging behaviours (e.g., self-injury, aggression, stereotypy, bizarre vocalizations) in individuals with intellectual disability (ID), often in combination. However, little is known about the behavioural mechanisms underlying psychopharmacological treatment. Establishing a better understanding of these mechanisms could contribute to improving treatment efficacy. For this study, I conducted repeated functional analyses using single-subject experimental designs to assess the impact of naturally varying dosages of psychotropic medications on behaviour function. Four individuals with ID who engaged in challenging behaviour and were undergoing psychotropic medication changes participated. Medication impact across two topographies for one participant, and three topographies for another participant were assessed, for a total of seven cases. For Analysis 1, I calculated standardized mean differences between baseline and final drug administration phases to estimate the overall effect of medication. I used this information to examine whether response rate following drug administration was related to response rate during baseline, referred to as rate-dependency. Rate-dependency was not observed. Analysis 2 explored the relation between psychotropic medications and behaviour function identified through functional analyses. Challenging behaviour was the dependent variable, while functional analysis conditions and psychotropic medication level served as independent variables. The latter was a quasi-experimental variable given participants’ psychiatric team prescribed changes independent of the researchers. Behaviour function correspondence, defined as no function change after a medication manipulation, was observed across 14 of the 21 medication manipulations (67%).