Arsenic has been known since time immemorial as a toxic substance. It is best known for its acute toxicity, for example when white arsenic is administered as a lethal potion. Chronic exposure to lower concentrations can likewise lead to dramatic situations, but in this case the poisoning develops very gradually.

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The word arsenic is thought to have derived from the Arabic al-zarnik, meaning orpiment (auri pigmentum of the Romans; in Greek: arsenikon). When a chemist talks of arsenic, he probably means the pure element arsenic (As). When a toxicologist talks about arsenic, he probably means arsenic oxide or white arsenic (As₂O₃). Pure metallic arsenic is not that poisonous, but its white salt arsenic oxide is very poisonous. The lethal dose of arsenic oxide for an adult is about 120-200 mg. Arsenic causes toxicity by combining with sulfhydryl groups (SH-groups) present on several enzymes and thereby blocking their action. Pentavalent arsenic can imitate phosphorus and replace it in the backbone of DNA, resulting in conformational changes and strand breakage.