Prescribing pharmacotherapy for older individuals with an intellectual disability (ID) is a complex process, possibly leading to an increased risk of prescription errors. The objectives of this study were (1) to determine the prevalence of older individuals with an intellectual disability with at least one prescription error and (2) to identify potential risk factors for these prescription errors (age, gender, body mass index (BMI), frailty index, level of intellectual disability and living situation). The study population consisted of 600 older ([greater than or equal to]50 years) individuals with an ID using one or more drugs who were randomly selected from the study cohort of the Healthy Ageing and Intellectual Disability (HA-ID) Study. The medication used at the time of measurement was screened for errors by a hospital pharmacist/clinical pharmacologist and a Master's student pharmacy using consensus methodology. Participants with one or more prescription errors were compared to participants without prescription errors by multivariate logistic regression to identify potential risk factors. The prevalence of individuals with one or more prescription errors was 47.5% (285 of 600 individuals; 95% confidence interval (CI) 43-52%). Relevant errors, defined as errors that actually do require a change of pharmacotherapy, were identified in 26.8% of the individuals (161 of 600 individuals; 95% CI 23-30%). Higher age (adjusted odds ratio (OR[subscript adj]) 1.03; 95% CI 1.01-1.06), less severe intellectual disability (moderate: OR[subscript adj] 0.48; 95% CI 0.31-0.74 and severe: OR[subscript adj] 0.56; 95% CI 0.32-0.98), higher BMI (OR[subscript adj] 1.04; 95% CI 1.01-1.08), higher frailty index (0.39-0.54: OR[subscript adj] 2.4; 95% CI 1.21-4.77 and greater than or equal to 0.55: OR[subscript adj] 3.4; 95% CI 1.03-11.02), polypharmacy (OR[subscript adj] 8.06; 95% CI 5.59-11.62) and use of medicines acting on the central nervous system (OR[subscript adj] 3.34; 95% CI 2.35-4.73) were independently associated with the occurrence of prescription errors. Interventions targeted to high risk patients should be designed and implemented to improve pharmacotherapy in older individuals with an intellectual disability. (Contains 5 tables.)