Poison

Poison Vial is a potion that can be created using the Alchemy skill.
 * Drag and drop on a weapon or a missile to apply the effect.
 * Countered by Antidote
 * Can also coat 20 arrows

Types of Poison Vials
Looks like damage and duration were changed some time after Sep 2014.

Links
Link 1 - Alchemy (as at 14 Sep '14)