Chlorpromazine equivalents and percentage of British National Formulary maximum recommended dose in patients receiving high-dose antipsychotics

Abstract

Aims and method: The study compared two methods of identifying patients prescribed high doses of antipsychotic medication among in-patients and out-patients in Buckinghamshire. High doses were defined as those in excess of 1000 mg chlorpromazine equivalents (CPZE) per day or more than 100% of the maximum recommended dose in the British National Formulary (BNF). Results: A total of 258 patients were identified as being prescribed antipsychotics, 30 of whom received high doses. There was 93% concordance between the two methods for identifying these patients. Nine of the 12 patients who had been prescribed high potency (> 1000 mg CPZE/day) but low toxicity (> 100% BNF maximum) therapy were receiving flupenthixol decanoate. A group of six high-risk patients were identified who had been prescribed lower potency drugs at supra-maximal doses. Clinical implications: Expressing total antipsychotic dose as a percentage of the BNF maximum is easy to understand and calculate. It helps to ensure patients at risk of antipsychotic toxicity are easily identified to ensure monitoring guidelines can be followed.