Moreover, multiple platforms offer testing of several polymorphic variants for which guidelines and labeling do not exist. Examples include variants in the serotonin transporter (SLC6A4), HTR2A, serotonin2 receptor (HTR2C), catechol-O-methyltransferase (COMT), and DRD2, as well as others. Variants in these genes have been extensively studied as candidates that may influence drug response, psychiatric disease risk, or disease-related phenotypes. Evidence exists to support hypotheses that these variants may influence response or adverse effects to psychiatric medications. However, the effect sizes for these outcomes seem to be small to moderate at best, with heterogeneity across studies and patient populations that needs to be resolved before clinical application becomes widespread. These statements of effect size and heterogeneous results may arguably be applicable to studies of drug metabolism markers. However, variation at these loci, which are more related to drug pharmacodynamics, have also been associated with disease risk, personality traits, or other psychiatric phenotypes that require important consideration as they relate to what the results may mean to the patient and their relatives.
