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Poison is any substance which acts in a pathological manner. The definition of poison is complicated, identified by the ancient Greek philosopher and scientist Paracelsus, who stated that "all substances are poisons, dependent on their dosage." Medically this is the general consensus also. Practically, however, many substances are of course innocuous in a low enough dosage - indeed one encountered in day to day life. Examples here would include nitrogen, chalk, water, etc. (though water toxicity may occur following hypothalamic compromise such as after taking noradrenaline (norepinephrine in the USA) reuptake inhibitors such as MDMA/Ecstasy). Other substances are very damaging in minute doses - notably the irreversible acetylcholinesterase inhibitors such as VX and Tabun (now employed in chemical warfare), heavy metals such as lead and mercury.

It is erroneous and extremely misleading and confusing in a clinical setting to use the words "poison" and "toxin" interchangeably. Poison is defined above, whereas a toxin is a poison derived from an organism. For example; excessive urea in the blood (from renal injury) and snake venom are toxins, whereas cadmium is a heavy metal poison but not a toxin. This error includes describing a poisonous substance as being "toxic".