A poisoned monster takes damage at the end of the monster's turn. The damage is Harm element, does not receive bonuses from your Stats, has no Type, and always hits. The damage is based on the poison's Level:
Base: (round((5.25 + 0.5625*Level + 0.00375*Level^2)/2) * 0.1
Rand: (round((10.5 + 1.125*Level + 0.0075*Level^2)/2) + ((round(200 + 13.2*Level)/200) * [2.1462*Level + 10.399]† / 8))* 0.1
†Minimum 10, maximum 200, rounded to the nearest 5
Poison does not receive bonuses from Stats.
If a monster is poisoned a second time, then the new poison either does nothing (if the new poison is of lower level) or replaces the old poison (if the new poison is of equal or greater level).