Based on literature review the paper presents some clinical aspects of the genetically determined polymorphism of the CYP2D6. One of the main biotransformation processes of psychotropic drugs is oxidation catalysed by enzymes of cytochrome P-450. CYP2D6 is an isoenzyme of cytochrome P-450. Its activity is determined genetically and is characterised by interindividual variability. Genetically determined polymorphism of CYP2D6 is related to mutated alleles that code enzymatic proteins with different activity. Based on individual ability to oxidize drugs by CYP2D6 in population there are four phenotypically different groups: extensive (EM), ultra-rapid (UM), intermediate (IM) and poor metabolizers (PM). Each phenotype is determined by a given genotype. About 6-10% of the Caucasian population is known as PM phenotype. Drugs used in standard doses in this group may reach a markedly higher level in blood, even a toxic level. Compared to the group with EM phenotype persons with PM or IM phenotype are more likely to suffer from side effects that are related to impaired metabolic pathways that are catalyzed by CYP2D6. In the group with UM phenotype (1-7% of population) metabolism is very rapid, thus they need higher doses of psychotropic drugs to reach therapeutic blood level of drug.