Polymorphism of P450 2D6 (CYP2D6) gene is one of the causes of altered activity of enzymes controlling metabolism of medicinal products (MP), specifically increasing and decreasing biotransformation of xenobiotics. These changes result either in a rise or in a fall of plasma MP; in the former case adverse effects (AE) of MPs are likely to arise, in the latter xenobiotic therapy may prove inefficient. Drug interactions also contribute to variation of MP levels in plasma and undesirable changes of their pharmacological action that may require correction of the therapeutic regime. This paper presents results of original studies and reviews domestic and foreign publications on polymorphism of genes encoding isoenzymes involved in biotransformation processes, such as CYP2D6. This issue is of importance for psychiatric and psychosomatic practice. Algorithms for the rational choice of antidepressants and antipsychotics are proposed with a view to enhancing efficiency and safety of therapy with the use of these MPs.