Pharmacogenetics and Pharmacogenomics constitute an especial young field of research in the domain of pharmacology. Both work on genetic variations which occur in individuals resulting reduced drug efficacy and more adverse drug reactions. Pharmacogenetics, emphasizes the diversity of patients and their genetic background, set their response to a given drug therapy, making understood the biological variability whereas pharmacogenomic considers the effects they cause in an individual (patient) different medications. The differences are studied on gene expression induction and repression of genes. The drug metabolism in human body is acted out by the P450 enzymes. In mammals, xenobiotic metabolizing CYPs provide crucial protection from the effects of exposure to a wide variety of chemicals, including environmental toxins and therapeutic drugs. In Phase I reactions (oxidation, reduction, and hydrolysis) and phase II conjugation reactions of drug metabolism in human body (acetylation, glucuronidation, sulfation, and methylation) are influenced by a number of genetic polymorphisms. Because many of the polymorphisms causing adverse effects are the result of single nucleotide changes, then a SNP profile of an individual could be used to guide therapy. If the genotype of an individual was known in advance then better clinical decisions could be made. Currently, more than 30 families of enzyme complexes responsible for drug metabolism have been described in humans and numerous variations exist in the encoding the many enzymes and proteins. The interdependence of the genetic makes up a large pharmaceutical research field today. More new analysis techniques beyond the understanding of the disease is possible and the Pharmaceutical treatment as targeted and individualized therapy. The ultimate goal of studies is to make more effective and affordable (financially) regimens with fewer side effects and greater patient response.