If you just walk it off, the poison will eventually go away on its own (or the character will die, which has the same net result :).
I've just done some tests with the spiders on level 3, with rather interesting results. At 8 points of poison resistance, I took an average of 84 HP of damage; with 58 points of poison resistance, I took an average of 35 HP of damage.
Some other notes:
- Poison damage is dealt once every 8 seconds, more or less. It seems that with high poison resist, you will sometimes take 0 damage on a tick; if this happens, you won't see anything change.
- At a constant level of poison resist, it seems to take a constant amount of time for the poison to clear out. For example, with 8 poison resistance, it consistently took 23 "poison ticks" in order to not be poisoned any more; if I put +50 poison resistance (58 total) bracers on my character, it consistently took 21 ticks for the poison to go away. However, this may just be because I took 0 damage twice and didn't notice. Hmm. This could be tested by measuring time elapsed, I guess.
- The amount of damage you take per tick appears to be uniformly random; unfortunately, it doesn't go down over time :(
- Saving after being poisoned and reloading will result in a different amount of damage being done to the character; the game is probably rolling dice per tick in the background :)
- Adding poison resistance after being poisoned helps just as much as having poison resistance when poisoned - so if you've got one poisoned character, stack all your poison resist on them.
Here's my raw data if anyone wants to look at it. I noted where I think I took 0 points of damage with "0?".