Interindividual differences in response to drug are defined as Pharmacogenetics / Pharmacogenomics. Vogel in 1959 first proposed the term “Pharmacogenetics. Over 50 years down the lane examples of exaggerated responses to drugs, novel drug effects, or lack of effectiveness of drugs as a manifestation of inherited individual traits have been observed. Genetic factors influence a drug’s action by affecting pharmacokinetic and pharmacodynamic properties. Unexpected, uncommon, or “abnormal” effects of drugs may be associated with certain genetically transmitted disorders. Under these circumstances, the modified drug response may have both diagnostic and therapeutic implications. These interindividual differences in response to drug are determined by combination of different factors; Physiological factors (sex, age); Pathological factors (liver disease, renal disease), Environmental factors (other drugs, diet, smoking), Genetic factors. How important each of these factors is, varies from drug to drug and individual to individual. So, pharmacogenetics explores the genetically determined alterations in the drugs usual metabolic pathways and these alterations are associated with the accumulations and toxicity of a drug and shifts to different pathways that have toxic intermediates.