In this episode, we review the science behind genetic differences in humans in the CYP2D6 hepatic enzyme responsible for drug metabolism and how these genetic variants can lead to certain drugs being metabolized far too much or far too little, which can cause drug toxicities or a lack of effectiveness. Key Concepts About 20-25% of drugs on the market are metabolized by CYP2D6. Humans have a huge degree of variability in CYP2D6 metabolism ranging from “ultra” metabolizers to “poor” metabolizers. Drugs that heavily rely on CYP2D6 metabolism are prone to large variability in responses due to these genetic differences. Some drugs rely on metabolic inactivation of CYP2D6 whereas other drugs use the enzyme to become converted to a more active compound. Codeine and tramadol both heavily rely on CYP2D6 activation to a more potent opioid compound. Patients with excessive CYP2D6 activity will have toxicities (from too much of an active metabolite) whereas patients with low CYP2D6 activity will have little therapeutic effect. Numerous antidepressants (paroxetine, nearly all tricyclic antidepressants, and venlafaxine) rely on CYP2D6 metabolism. Differences in CYP2D6 metabolism have been shown to either cause toxicity or a lack of effectiveness with these medications. References Chartrand R, Forte AM, Hoger JD, Kane SP, Kisor DF. Pharmacogenomics and Commonly Prescribed Medications. AdvanCE. October 10, 2022. https://www.advancepharmacist.com/courses/pharmacogenomics-and-commonly-prescribed-medications. Caudle KE, Sangkuhl K, Whirl-Carrillo M, et al. Standardizing CYP2D6 Genotype to Phenotype Translation: Consensus Recommendations from the Clinical Pharmacogenetics Implementation Consortium and Dutch Pharmacogenetics Working Group. Clin Transl Sci. 2020;13(1):116-124. doi:10.1111/cts.12692 Bousman CA, Stevenson JM, Ramsey LB, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6, CYP2C19, CYP2B6, SLC6A4, and HTR2A Genotypes and Serotonin Reuptake Inhibitor Antidepressants [published online ahead of print, 2023 Apr 9]. Clin Pharmacol Ther. 2023;10.1002/cpt.2903. doi:10.1002/cpt.2903 Crews KR, Monte AA, Huddart R, et al. Clinical Pharmacogenetics Implementation Consortium Guideline for CYP2D6, OPRM1, and COMT Genotypes and Select Opioid Therapy. Clin Pharmacol Ther. 2021;110(4):888-896. doi:10.1002/cpt.2149