Poison

Poison is a potion effect that continuously drains. Poison deals, meaning it isn't affected by. This effect cannot kill you; it will stop at the vanilla equivalent of a half heart. The Poison effect can be obtained by:
 * Splashing yourself with or drinking a Poison Potion
 * Getting splashed with a Poison Potion from a Witch
 * Getting hit by a Cave Spider, either spawned by the Broodmother or a Cave Spider Minion

The Vaccine Talisman and the Cow Head provide immunity from the poison effect. Not to be confused with the debuff inflicted by Arachne and Arachne's Keeper.