Classical toxicology deals with the effects of chemicals on humans and domestic animals under conditions of their direct exposure. Experiments have shown that poisoning requires interaction between poisons and the organism, i.e., their entry into the organism, poisoning occurring only when the dose of a harmful substance attains a value at which deleterious effects appear. Depending on the amount of a given substance, it can be indifferent, medicinal, or poisonous in its action on the organism. For example, the well – known poison arsenic is used as a medicine in small doses, while many medicines become poisons with a significant increase of dose.Poisons enter the organism by means of inhalation and ingestion or through the skin. On penetrating a living cell, poisons alter physico – chemical characteristics of the cytoplasm, destroy the membranes of organelles, change the reaction of the cell medium, and disturb the conditions needed for normal functioning of cell proteins. Enzymes – cell biocatalysts – are especially sensitive to the action of poisons. Poisoning of some enzyme that takes part in an important metabolic process in the final analysis has a lethal outcome. Poisons act as enzyme inhibitors, either general (salts of heavy metals such as Ag, Cu, Hg, and Pb) or specific (cyanides, H2S, sulfides, azides, and CO, which act on metals).