Poison is a status effect in Salt and Sanctuary. You can inflict poison on enemies using weapons with poison effects, spells/charms/buffs that inflict poison, or items that inflict poison.
Poison slowly does damage over time once inflicted. The speed of the damage ticks increase the more you poison your foe, which is made easier with higher poison damage weapons and effects. Essentially, poison 'stacks' in this way through allowing you to continue 'adding' poison to your enemy as you hit them.
The weapons that inherently inflict poison are:
You can add poison to your weapon through:
Poison Gas is an incantation that can poison enemies without the use of a weapon.