Precision medicine applies primarily to pharmacokinetics in toxicology. Mastering hepatic metabolism through an understanding of the genetics behind Phase I, or oxidation/reduction and some Phase II, or conjugation, enhances the scientific and clinical application of common drug toxicology. This review includes basic hepatic metabolism, the common substrates, inducers and inhibitors of cytochrome P450 along with genetic variants behind the enzymes. Detailed metabolism of commonly measured opioids in clinical practice is provided. Finally, evidence based research and clinical correlations conclude that knowledge of inducers and inhibitors, in conjunction with genetic variations, are integral components for applied precision medicine in toxicology.