Drug metabolism refers to the biochemical transformation of a compound into another more polar chemical form. Absorption, distribution, metabolism and excretion comprise an integral part in understanding the safety and efficacy of a potential new drug. Detailed in-depth knowledge of the Pharmacokinetics and Drug Metabolism of a new drug entity is considered a prerequisite to know the appropriate route of administration, correct dose etc. Sometimes there is (are) different / unwanted effect(s) of certain drugs in different populations. This is particularly true for the drug having narrow therapeutic index. Often these different effects are detrimental to an individual, thus termed as adverse drug reactions. After the raw draft of human genome has evolved, it has become increasingly clear that change(s) in the drug response between individuals, is due to the occurrence of genetic polymorphisms in the Phase I and II drug metabolizing enzymes, due to which distinct subgroups in the population differ in their ability to perform certain drug biotransformation reactions. The study about the occurrence of genetic polymorphisms in drug metabolizing enzymes is termed as Pharmacogenetics / Pharmacogenomics. Pharmacogenetic characterization of particular drug can be both phenotypically or genotypically conducted in population groups. The study is very important to check the post-marketed drug withdrawal, if a particular percentage of population suffers from adverse drug reactions, and thus a lot of revenue be saved. The study also helps to find out Right Medicine for Right Individual or Individualized Medicine.