The objective of the present work was to develop the forensic medical criteria for chronic narcotic intoxication based on the results of morphological studies. The internal organs from 179 cadavers were available for the examination following death from acute poisoning with narcotic substances or as a result of chronic narcotic intoxication. The studies were carried out with the use of routine histological staining techniques and an immunohistological method. The data obtained provided a basis for the development of criteria to be employed in forensic medical diagnostics of acute poisoning with narcotic drugs and chronic narcotic intoxication. These criteria include ischemia of cerebral neurons, pulmonary emphysema with the formation of foreign body-type granulomas and fibrin/erythrocyte thrombi, morphological signs of ventricular fibrillation, the picture of bacterial endocarditis, follicular hyperplasia of the lymphoid organs, chronic portal hepatitis, and nodular degeneration of the adrenal cortex associated with its atrophy.