The article describes examples of pharmacologic interaction of drugs and biologically active substances of medic-inal plants (phytopreparations) at various stages of metabolism. In pharmacotherapy, phytopreparations have proved as the safest low-toxic medicines so they often prescribe to children, elderly, women during pregnancy and lactation. Often patients resort to self-medication with phytotherapy to enhance the therapeutic effect of pre-scribed medicines, without thinking that active plant components can affect the drug. For example, the combined use of expectorants and phytopreparations, having expectorant and mucolytic action (marshmallow, thyme, ivy, etc.) leads to increased expectorant action. Contraindicated warfarin and other anticoagulant with phytoprepara-tions from chestnut horse, gingo, St. John's wort, since there is a bleeding disorder. Depending on the interaction of drugs with phytopreparations, medicines should be taken together or separately with phytopreparations. The at-tending physician or pharmacist should advise the patient about the advisability of combining phytopreparations with drugs.