Metabolomics, the study of metabolism at an "omic" level, has the potential to transform our understanding of mechanisms of drug action and the molecular basis for variation in drug response. It is now possible to define metabolic signatures of drug exposure that can identify pathways involved in both drug efficacy and adverse drug reactions. In addition, the "metabotype," the metabolic "signature" of a patient, is a unique identity that contains information about drug response and disease heterogeneity. The application of metabolomics for the study of drug effects and variation in drug response is creating "pharmacometabolomics," a discipline that will contribute to personalized drug therapy and will complement pharmacogenomics by capturing environmental and microbiome-level influences on response to drug therapy. This field has the potential to transform pharmacology and clinical pharmacology in significant ways and will contribute to efforts for personalized therapy. This overview highlights developments in the new discipline of pharmacometabolomics.