Poison

Poison is a potion effect that continuously drains  Health. Poison deals ✷ True Damage, meaning it isn't affected by  Defense. 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 Hat provide immunity from the poison effect. Not to be confused with the debuff inflicted by Arachne and Arachne's Keeper.