Drug dose efficacy/toxicity depends on genetic/allelic variations and nongenetic cues including a pre-existing disease, hormonal balance, life style, diet, and other factors including concurrently administered drugs. Cytochrome P450 (CYP) proteins play a crucial role in drug metabolism where they catalyse a number of Phase I oxidation reactions. Alteration in CYP activity due to genetic or allelic variations and other factors can significantly alter therapeutic outcome. It can convert an extensive metabolizer for a particular drug into a poor metabolizer phenotype (phenoconversion), resulting in treatment failure or toxicity. Hepatic CYP is particularly important as it metabolizes about ¾ of all drugs. Determining the actual CYP activity by calculating the plasma drug metabolite and drug ratio (in vivo phenotyping) gives a better understanding of true activity especially in cases of clinically important CYP isoforms commonly reported in drug oxidation reactions such as CYP3A4/5, CYP2C19, CYP2C9, CYP2D6, and CYP2B6, one of the most polymorphic human CYP known for its high inter-individuals and within-individual variability and involvement in the metabolism of a number of common drugs (artemisinin, bupropion, cyclophosphamide, efavirenz, ketamine and methadone). In vivo phenotyping of clinically important CYP isoforms may provide a valuable tool in the hands of advanced physicians to optimize therapeutic dose and manage unpredictable treatment outcomes or treatment failure and drug toxicity cases and can better equip a physician for precision or personalized therapy in vulnerable individuals. This review highlights the variations in CYP activity due to various reasons and the importance of in vivo phenotyping over genotype in ascertaining drug bioavailability and dose optimization, implicating the determination of plasma (drug) metabolite/drug ratio in an individual as an important approach in predictive personalized medicine.