Chlorpromazine equivalents versus defined daily doses: How to compare antipsychotic drug doses?

Classic chlorpromazine (CPZ) equivalents can be used to chart relative antipsychotic potencies of antipsychotic drugs. Values of CPZ equivalents per drug are ambiguous in literature. In drug use evaluation studies, antipsychotic doses are frequently compared by use of the defined daily dose (DDD). The DDD is the assumed average maintenance dose per day for a drug if used for its main indication in adults. The DDD is based on review of the available older and recent literature. In this report, we evaluated discrepancy between CPZ-equivalent values and DDD-equivalent values. We plotted CPZ-equivalent values against DDD-equivalent values and performed linear regression to determine the mean relationship between the 2 methods. About 67% of the DDD-equivalent values demonstrated lower potencies for antipsychotic drug compared with CPZ-equivalent values. The slope of the regression line was 0.68 (r(2) = 0.81). Because we found a great discrepancy between these 2 methods of comparing antipsychotic drug doses, we think further research is necessary to develop a standardized way of antipsychotic drug comparison.