Poison has a rating the same as the HD of the creature that it came from (ie. a Purple Worm has an HD 15, so its Poison is strength 15). If the poison is rated higher than your level, then you save vs. poison or die. If it is equal to or lower than your level then you save or take damage for a number of rounds equal to its rating (unless an antidote or Cleric gets to you first).
If the poison has some effect other than or instead of damage (like makes you hallucinate, or vomit, or fall unconscious) then that last for a number of rounds/turns equal to the HD if it’s greater than your level, or half HD if it’s under your level.
Either way, thieves can add their level to their saving throw.
Poison can be created by successfully skinning the venemous creature. Generally a creature yields 1d6 doses of venom which can be turned into the poison or the antidote for that specific poison. Artificial and unnatural poisons concocted by nefarious asshats can have a rating of whatever you want, obviously, or you can base it off the level of the creator. That way your NPC enemy Master Thief Radagan, purveyor of fine poisons can have 10 rated poison, but you know Dug Dug the Thug Thug only have poisons at 4.
Thiefy and Rangery types can identify poison (either by inspecting the poison itself, or by seeing a victim afflicted by it) by rolling under their INT+level. Though I wouldn’t give rangers a chance to identify manmade poisons, unless there’s a chance they would spot one of the natural ingredients.
After awhile your PCs could actually have their own lists of what the types of poisons are and how strong they are. Thieves can be a walking cornucopia of poisons and antidotes.
My next step would be to come up with a list of side-effects and symptoms for the players (i.e. You feel your throat constrict and start to hallucinate, or the Ranger’s veins pump purple and his eyes roll back into his head, tastes like pennies, smells like ammonia, has to be imbibed, works on touch, works on cuts only, that kind of thing). But there’s already a decent amount of lists like that out there.
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