Developing a Resistance to Poisons
A character could conceivably develop a resistance to specific poisons. These are the steps they must take:
Prerequisites: A character MUST either have 1 rank in BOTH Craft (Alchemy) and Heal or have the Poison Use class feature (such as the Assassin has). This simulates the knowledge acquired in poisons not to kill one’s self outright during this process.
The character must then actually take the poison in question multiple times. They will take 1/2 of the consequences of taking the poison. All successful saves will be counted. So, if a character is working on a resistance to Lich Dust and they miss their save, normally they would suffer d3 STR. However, they only suffer a d2 in this case.
They must MAKE a number of saves equal to 10 minus their CON modifier to receive their +1 resistance to that poison. So, if a person has an 18 CON, that have a +4 modifier, they must make 6 saves versus that poison to get their +1 modifier.
This process must be completed within one month. The process can be repeated for a maximum of a +5 modifier.