Some people devote considerable time and attention to the study of one particular form of poison. They can make the poison, provided that they have the raw components. They intentionally expose themselves to the poison, bit by bit, over time, giving them a natural immunity to that specific poison.
The DM decides what type of poisons are available to the character, and their duration (in the event of paralytic or time-delayed poisons), as well as any pluses or minuses to the Saving Throw.
When creating the poison, they must make a successful proficiency check. If the check fails, then they are unable to make the poison. If they role a natural 20 (an automatic failure), then the DM must successfully make a percentile role. There is a base 24% chance, minus 1% per level of the character, to accidently create an extremely lethal poison that is dissimilar enough from the intended poison that the character may kill himself by exposure (treat as type N, touch poison). The percentage chance is never less than 2%. A seventh level rogue that rolled a 20 for his proficiency check would have a 17% chance of creating just enough lethal poison to kill himself; there is no save, for this type of poisoning (the reasoning being that the new poison is also similar enough to the old that the character's body undergoes a lethal allergic reaction).
Next Proficiency of the new Proficiencies