The cytochromes P450 (CYP) enzyme system is involved in the activation and inactivation of drugs. Genetic defects in these genes can alter drug metabolism and create interindividual differences in drug efficacy, toxicity and drug dependence. We investigate these interindividual differences using molecular genetic, pharmacogenetic and pharmacokinetic approaches, using in vitro, animal, epidemiological and clinical treatment studies. In addition, CYPs are present in the central nervous system where they are regulated by drugs of abuse (e.g., alcohol and nicotine). We investigate the influences of drugs of abuse on brain CYPs, integrating the fields of drug metabolism and receptor neuropharmacology, in order to identify interindividual differences in neurotoxicity and drug response. Our experimental approach principally involves animal behaviour, biochemical, pharmacokinetic and molecular biological techniques.