The National Institute on Drug Abuse (NIDA) supports research and development of new medications for the treatment of addiction. The medications development program of NIDA involves in vitro evaluations, preclinical pharmacology and toxicology studies, pharmaceutical development, and clinical evaluations for potential new medications for treating addiction. The body generally eliminates drugs either by excretion or by metabolism to one or more active or inactive metabolites. Metabolic pathways, individual differences in metabolic rates, and genetic polymorphism in enzymes can significantly affect safety and efficacy of the drug. It is important to understand these metabolic pathways and potential interactions at an early stage of drug development. One can characterize drug metabolism using simple, rapid, and low cost in vitro methods to provide predictions of toxicity and efficacy which may significantly affect the design (or initiation) of clinical evaluations.