Upon entering a room, creates a green fog effect and all enemies are poisoned.
The poison ticks 2 times.
The poison ticks 4 times on normal enemies and 2 times on bosses (likely a bug).
Killing enemies leaves a small, short-lasting pool of greencreep on the ground.
The creep deals 1 damage per tick or 30 damage per second.
Notes
High tear damage will cause this item to become very powerful for rooms with a lot of enemies, since they can die instantly and render the room harmless.
Enemies which spawn after the first few seconds of each room are not affected by this item, e.g. Flies spawned from a Mulligan dying.
Enemies that spawn with some form of invulnerability such as Hosts, Wizoobs, or Needles are not affected by this item.
The poison effect only activates upon entering a room and not at the start of a wave. In Boss Rush and most of Greed mode, the item will not poison enemies.
If Toxic Shock is picked up in the middle of a room with enemies, the mass poison will occur immediately.
Synergies
Second Hand: Extends the poison duration, allowing it to tick 6 times on normal enemies and 4 times on bosses.
The poison ticks 2 times.
The poison ticks 4 times on normal enemies and 2 times on bosses (likely a bug).
Mulligan dying.
Hosts, 
Wizoobs, or 
Needles are not affected by this item.
Boss Rush and most of Greed mode, the item will not poison enemies.
Second Hand: Extends the poison duration, allowing it to tick 6 times on normal enemies and 4 times on bosses.
Toxic Shock: Doesn't stack
Red Stew: High damage stat makes 
E. Coli/
Flush!: Enemies that are turned into poop don't leave creep.
Frozen enemies do leave creep.
Contagion is a recolor of its sprite.
The Binding of Isaac: Rebirth
