- Gives Isaac a chance to shoot poison tears.
- Poisons enemies that make contact with Isaac.
- Enemies affected by the poison have a chance of dropping black hearts.
- Athame: The black ring triggered by Athame applies the Serpent's Kiss' poison. Enemies killed by the ring will have an increased chance to drop black hearts.
- Invulnerability: Contact while invincible deals poison damage.
- Scorpio: Grants a 100% chance of poison tears.
- Second Hand: Increases the duration of the poison.
- Soy Milk: Increases the chance of enemies and bosses being poisoned due to the low damage and high fire rate.
- The Common Cold: Adds to the chance of firing poison tears, the chance increasing with Isaac's luck.
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