The effect of pharmacogenetic testing for CYP450 2D6 and 2C19 on treatment costs have not yet been documented. This study used Danish patient registers to calculate health care costs of treating patients with diagnoses within the schizophrenic spectrum for one year with or without pharmacogenetic testing for polymorphisms in the genes for the CYP2D6 and CYP2C19 enzymes. In a randomized, controlled trial, stratified with respect to metabolizer genotype, 104 patients were assigned to treatment based on pharmacogenetic testing and 103 patients to treatment as usual. Random exclusion of extensive and intermediate metabolizers was used to increase the frequency of extreme metabolizers (poor metabolizers and ultrarapid metabolizers for CYP2D6) to 20% in both groups. This article is protected by copyright. All rights reserved.