Buttercup: And to think, all that time it was your cup that was poisoned.
Man in Black: They were both poisoned. I spent the last few years building up an immunity to iocane powder.
— The Princess Bride
Named after Mithridates, a Persian king with a morbid but well founded fear of being poisoned, mithridatism is a form of accquired immunity to poisons that develops as a result of taking sub lethal doses of various poisons over a prolonged period and thus developing a tolerance.
In fiction, this is effective against all poisons and can be easily and safely accquired, usually as part of career progression for assasin and alchemist type characters. Ranger and Druid types may also accquire immunity to 'natural poisons' … usually meaning venoms and poisonous plants.
In reality, this effect is hard to pull off successfully - a lot of poisons aren't subject to immunity and cause permanent harm in relatively small doses and/or build up in the body over time, not only that but building immunity to more than one poison at a time could result in the poisons interacting with one another and having a more severe effect as a result. Also the user must contend with the negative effects of even sub-lethal doses of poisons and must keep himself dosed to maintain immunity. Immunity to venoms, on the other hand, is a relatively well established phenomenon - although, as always, not all venoms are subject to tolerance.
Mithridates, however, is said to have protected himself quite effectively from at least one assasination attempt using poison … only to find one last problem with his accquired immunity: when he had lost the Pontic War and fell into Roman captivity, his attempts to commit suicide were greatly hampered.
Game and Story Use
- In a fantasy campaign (as noted) something similar may be either a feat or edge or part of some class's career progression.
- May be a good trait for an NPC who PCs are trying to poison.
- Or used as an excuse for some kind of supernatural immunity - perhaps the NPC is actually undead and hiding his true nature with magic. If there are poisons widely known to bypass mithridatism one of these might prove diagnostic.
- Tolerance to the venom of a specific type of creature is (relatively) realistic for someone who has spent a lifetime working with them … of course this may be cause or effect. Perhaps if they weren't so immune they would have died or quit ages ago.