ADME genes are a group of genes that carry information for proteins involved in the absorption, distribution, metabolism and extraction of drugs, toxins and other chemical substances from the environment. For some patients the same medicine works faster for others it works more slowly or it can cause more or less side effects, even death. Due to the existence of a large number of pharmacogene variants, protein variants can have different activity and metabolizing rates leading to an individual specific variable response of the organism to a certain drug. Administration of an effective drug in an appropriate dose becomes a challenge of pharmacological therapy. The specific distribution of ADME genes variations among people is used in research on the relationship between genetic and geographical distance of populations, ethnicity and the evolutionary past of the population. Roma as an isolated transnational population originating from India with a pronounced founder effect, endogamy and reproductive isolation are a model in the study of genes whose variability changes between populations such as the ADME genes. In Croatia, this population is divided into the Balkan (Zagreb), Međimurje and Baranja subpopulations. By researching the position of Croatian Roma with regard to the worldwide variation of the ADME gene locus, the frequency of the rarer allele, the relationship between genetic variation and geographic distance, and the intra- and interpopulation analysis of CYP2B6, CYP2D6, CYP2C19 and SLC22 ADME gene polymorphisms and their phenotypes an effort is made to contribute to the selection of a suitable drug for effective therapy for members of this population.