People talk of poison as if it were some class of chemical distinct from all others. In actuality, any chemical substance can be poisonous to humans, if you are exposed to enough of it. Life is fragile, a delicate balance of chemical processes which can be thrown off by the unmonitored introduction of any chemical.
Those substances we call poisons are actually just those for which we have very low tolerances.

In 1957, Dr. William Coda Martin tried to answer the question: When is a food a food and when is it a poison? His working definition of "poison" was:
"Medically: Any substance applied to the body, ingested or developed within the body, which causes or may cause disease.
Physically: Any substance which inhibits the activity of a catalyst which is a minor substance, chemical or enzyme that activates a reaction."1 The dictionary gives an even broader definition for "poison": "to exert a harmful influence on, or to pervert"

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