Sold by the Undead Female Merchant for 100 Souls each and deal out Poison damage. Not all enemies can be poisoned and almost all take more than one hit to become poisoned. These are extremely useful for lowing the health of some large enemies or to thin out packs of them.
The physical damage the arrows do can be ignored as you will mostly likely be trying to poison the enemy from a distance. Once an enemy is poisoned, their health bar will appear. The distance the arrow travels has no affect on the poison infection.
Poison Arrows are most useful early in the game.
Enemies that can be poisoned will take 2 to 3 (or more) arrows until they are infected. Poisoned enemies take a long time to die, but sometimes the wait is worth it.