There are a huge number of things which can be considered poisons.
Many poisons are non-fatal in small doses, and it is possible to form an imunnity to many through prolonged exposure. Poisons are one of the many tools used by assassins.
A lack of scientific knowledge also lead to people unknowingly poisoning themselves. For instance, Elizabeth I is frequently portrayed with white face make-up that had a lead base. However, this cannot be credited as being the cause of her death.