long time use of Arsenic is a key plot point there. In the Angelique novels, our heroine's first husband, Joffrey, who is among other things a genius level scientist, has her consume a tiny dose of Arsenic on a regular leve, as he does himself, in order to immunize herself against being poisoned. It does save her life later on. But you really have to get the dose right to use it like that. (I think. I'm anything but a scientist.)

See also: Strong Poison by Dorothy Sayers! The bad guy takes small doses over a long time so that he can eat the same meal as his victim later on (the question of when the poison was administered and who did it being the key question in the case). The immunization does apparently work, but you can still expect some bad consequences for your health after a while, see this interesting blog post about the book and about arsenic poisoning in general by Deborah Blum, who wrote a whole book about poisons. /tangent