Paracelsus, sometimes called the father of toxicology, wrote:
German: Alle Ding' sind Gift, und nichts ohn' Gift; allein die Dosis macht, daß ein Ding kein Gift ist.
"All things are poison, and nothing is without poison; only the dose permits something not to be poisonous."
Or, more commonly
"The dose makes the poison."
That is to say, substances considered toxic are harmless in small doses, and conversely an ordinarily harmless substance can be deadly if over-consumed.