From Bioinformatics.Org Wiki
All individuals respond differently to drug treatments; some positively, others with little obvious change in their conditions and yet others with side effects or allergic reactions. Much of this variation is known to have a genetic basis. Pharmacogenetics is a subset of pharmacogenomics which uses genomic/bioinformatic methods to identify genomic correlates, for example SNPs (Single Nucleotide Polymorphisms), characteristic of particular patient response profiles and use those markers to inform the administration and development of therapies. Strikingly, such approaches have been used to "resurrect" drugs thought previously to be ineffective, but subsequently found to work with in subset of patients. They can also be used for optimizing the doses of chemotherapy for particular patients.