Most DMs and Players I have talked to agree that poisons in Pathfinder are next to useless by mid to high level due to the fact they are expensive and their DCs are easy to beat even in low levels.
However reading the poison rules Multiple doses of poisons section I ran across the rules on "stacking" poisons and needed some clarification.
Making your initial saving throw against a poison means stacking does not occur—the poison did not affect you and any later doses are treated independently. Likewise, if a poison has been cured or run its course (by you either making the saves or outlasting the poison's duration), stacking does not occur. However, if there is still poison active in you when you are attacked with that type of poison again, and you fail your initial save against the new dose, the doses stack. This has two effects, which last until the poisons run their course.
Increased Duration: Increase the duration of the poison by 1/2 the amount listed in its frequency entry.
Increased DC: Increase the poison's DC by +2.
I read this as you only get the DC increase AFTER you fail the original DC.
For example the following events happen:
- Hit with Black adder venom DC 11
- Pass your save
- Hit with Black adder poison again.
So on the 3rd step the DC would remain 11 as you passed your first save or would the DC stack up to 13?