Everything that is poisonous when used in one way, can be medicinal if applied in a different way. One could say, that all substances are both poisonous and medicinal. Of course, if you drink a few buckets of water in succession, it works like poison; If you drink it in the proper quantity, it is nutritious and if one drinks it in exceptionally small amounts, it can be used in a medicinal way. In general, through employing various methods, water has a potent healing ability. One can therefore say of most ordinary substances, that what is poison can also be medicine. Consequently, one must recognize how such a substance works, including the effect it has on the human constitution.
Source (German): Rudolf Steiner – GA 348 – Über Gesundheit und Krankheit – Dornach, January 13, 1923 (page 250)
Translated by Nesta Carsten-Krüger