A poison or toxin is a substance that can readily harm a living organism at a low dosage. Toxins can be a result from metabolic processes and can be absorbed by the body or come into contact with it. If a harmful substance is generally effective in nature, it is called an environmental toxin. The likelihood of health impairment as a result of poisoning increases with the exposure quantity of the active ingredient. Accordingly, almost every substance is classified as poisonous (toxic) as soon as it exceeds a certain dosage. 

The science that deals with the research of toxic substances, their effect at various doses and the treatment of poisoning is toxicology. It is concerned with the substances or substance mixtures and the biochemical mechanisms of toxic effects on animals, plants and microorganisms. The damage caused by a poison can be a temporary or permanent impairment or may even result in death. If the harmful effect permanently persists, it is called chronic poisoning. If the effect of poisoning immediately leads to impairment, this is called acute poisoning.