Background Although people with intellectual disability (ID) and people with dementia have high drug prescription rates, there is a lack of studies investigating drug use among those with concurrent diagnoses of ID and dementia. Aim To investigate the use of antipsychotics, benzodiazepine derivatives, and drugs recommended for dementia treatment (anticholinesterases [AChEIs] and memantine) among people with ID and dementia. Methods and procedures Having received support available for people with ID and/or autism spectrum disorder (ASD) was used as a proxy for ID. The ID cohort consisted of 7936 individuals, aged at least 55 years in 2012, and the referent cohort of age- and sex-matched people from the general population (gPop). People with a specialists’ diagnosis of dementia during 2002–2012 were identified (ID, n = 180; gPop, n = 67), and data on prescription of the investigated drugs during the period 2006–2012 were collected. Outcome and results People with ID/ASD and dementia were more likely than people with ID/ASD but without dementia to be prescribed antipsychotics (50% vs 39% over the study period; odds ratio (OR) 1.85, 95% confidence interval 1.13–30.3) and benzodiazepine derivatives (55% vs 36%; OR 2.42, 1.48–3.98). They were also more likely than people with dementia from the general population to be prescribed antipsychotics (50% vs 25%; OR 3.18, 1.59–6.34), but less likely to be prescribed AChEIs (28% vs 45%; OR 0.32, 0.16–0.64).