Genotyping for CYP2D6 and CYP2C19 variations is emerging as a potentially useful clinical tool to help mental health professionals prescribe psychiatric medications for their patients. Cytochrome P450 testing uses a blood sample to determine an individual's required dosage of identified drugs that are metabolized by the two enzymes. To provide care for patients and families, nurses should be able to demonstrate identified essential nursing competencies related to genetics and genomics, which include an understanding of cytochrome P450 testing. As patient advocates, nurses are expected to understand how to identify patients most likely to benefit from CYP2D6 and CYP2C19 testing, how to ensure informed consent for such testing, and how to educate patients about testing and test results.