Calculating cumulative equivalent doses of anti-psychotics
The use of “high doses” of anti-psychotics is associated with increased clinical risk to patients.
Despite the Royal College of Psychiatrists providing guidance on High Dose Antipsychotic Prescribing in 2006, there is still some confusion about how this is calculated.
The Royal College of Psychiatrists defined high doses as “a total daily dose of an anti-psychotic which is above the BNF limit or a total daily dose of two or more anti-psychotics which exceeds the BNF maximum using the percentage method.”
The percentage method requires the conversion of the dose of each anti-psychotic into a percentage of its BNF maximum dose, and then adding the percentages together. A cumulative dose of more than 100% is a high dose.
The BNF maximum dose for olanzapine is 20mg and for haloperidol is 30mg (orally). If a patient is prescribed olanzapine 15mg + haloperidol 15mg (PRN), the cumulative dose would be:
15/20 x 100% + 15/30 x 100% = 75% + 50% = 125%.
The Royal College of Psychiatrists recommends additional monitoring and review for patients who are prescribed high doses, and hospitals should have a policy for this.
Further details are available in the Royal College of Psychiatrists consensus statement:
Cumulative equivalent doses are often mentioned on the Mental Health Act treatment forms, so it is essential to know their significance to ensure treatment is legally compliant.
Cumulative equivalent doses are covered as part of our recently launched Mental Health Act clinical training seminar, details of which are outlined at the top of this page.
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