Poison and poisoning are important concepts in medicine. However, there is great difficulty to define it, the difficulty is that the poison itself does not have the essence different from the non-poison. Toxicity determined by dose is not a species difference, but exists in the relationship between the poison and the subject, and it makes no sense to escape from the poison alone to define the poison. Poison and poisoning have played a very important role as explanations of etiology and disease, and as medicine develops, this explanation is partially replaced by theories such as pathology anatomy, cytopathology and bacteriological theory, but as an alternative hypothesis, poisoning as disease and toxicant as etiology is still need to be retained.