A novel approach for evaluating and comparing multiple antipsychotic polypharmacy in adults across specialist intellectual disability services in England and Wales: A retrospective feasibility cohort study

Background: In England and Wales, significant numbers of people with intellectual disability (PwID) are prescribed antipsychotics for a range of licensed and non-licensed indications, particularly behaviours that challenge. There is no system to compare antipsychotic prescribing practices across specialist adult intellectual disability (ID) services. Aim: This study aimed to determine whether converting antipsychotics and their doses to equivalent chlorpromazine units enables comparison of licensed (psychosis, etc.) and non-licensed (behaviours that challenge) prescribing between different services. Method: A feasibility retrospective cohort study using convenience sampling captured anonymized data of PwID on ⩾2 antipsychotics across eight specialist England and Wales adult ID services. The doses prescribed across 2017–2023 were converted to chlorpromazine-equivalent values, and antipsychotic dose burden across services was compared. Patients’ characteristics and antipsychotic dosing are summarized descriptively. A mixed-effects regression model determines changes in chlorpromazine unit values across time. Subgroup analysis explores trends. Stata: Release 17 (StataCorp, 2021) and R (R Core Team, 2025) were used. Results: Of the 424 reviewed records, a higher proportion were male (ranging from 58% to 71% across census years) and white individuals (73%–85%). Autism (42%–58%), psychotic illness (61%–71%), and behaviours that challenge (77%–87%) were the most common comorbidities. Chlorpromazine units could be captured for 98% ( n = 416). PwID with psychosis, behaviours that challenge, depression, or attention-deficit/hyperactivity disorder were prescribed significantly higher chlorpromazine-equivalent doses. Over 25% were prescribed more than 1000 mg/day chlorpromazine equivalent, which exceeds the British National Formulary-recommended maximum dose. Conclusions: Our study suggests that converting antipsychotic doses to chlorpromazine units is a simple and effective way to understand and compare prescribing burden for PwID. It also facilitates measuring prescribing and implementing best practice.