This review provides the reader with different information regarding biomarkers, genetic biomarkers in psychiatric disorders, and the role of different cytochrome enzyme variants in the treatment of psychiatric disorders. A biomarker is a specific characteristic that indicates normal biological processes, pathogenic processes, or responses to exposure or intervention. According to their clinical use, there are several types of biomarkers, including diagnostic, monitoring, pharmacodynamic, predictive, prognostic, safety, and risk biomarkers. Biomarkers can also be categorized by “omics,” such as proteomics, transcriptomics, metabolomics, genomics, epigenomics, connectomics, and microbiomic biomarkers. Genes implicated in schizophrenia and depression include those involved in neurotransmitter systems, such as serotonin, dopamine, and glutamate; immune system functioning; and neurodevelopmental processes. Alzheimer’s disease (AD) is a complex disease that may involve several genes. Personality disorders have a more complex genetic architecture than other psychiatric conditions. Several enzymes, such as CYP1A2, CYP2B6, CYP2D6, CYP2C8, CYP2C9, CYP2C19, and CYP3A4, are involved in the metabolism of various psychotropic drugs. However, drug-drug interactions that can significantly affect clinical outcomes are often overlooked. Most interactions between antipsychotic drugs are due to their CYP metabolism, although pharmacodynamic interactions are also possible.