The Common Cold

Effects

Adds a chance to shoot poison tears, dealing damage over time when inflicted.
(Removed in Afterbirth) The poison randomly hits either 2 or 3 times, dealing a total of 4 or 6 damage.
- (Added in Afterbirth) The poison hits twice, dealing a total of double Isaac's damage.

The chance for a tear to be poisoned depends on the luck stat. The activation rate is equal to 1/(4 - (Luck / 4)). At base luck (0), the chance is 25%, maxing out at 100% at 12 luck.
- Due to how this formula works, increases in activation rate are only seen with every 4 points of luck, as this formula rounds down after dividing Isaac's luck by 4.

(Added in Afterbirth †) Compound Fracture/ Cricket's Body/(Added in Afterbirth †) Haemolacria/ The Parasite: Chance for tears and split tears to be poisonous. If the main tear is poisonous, the split tears are too.
Dr. Fetus/ Epic Fetus: Chance for bombs/explosions to become poisonous like Bob's Curse.
(Added in Afterbirth †) Finger!: Finger can also apply the poison effect.
(Added in Afterbirth †) Lachryphagy: If a poison tear is fed, all burst tears are poisonous. If a normal tear is fed, the burst tears are not poisonous.
The Ludovico Technique: The tear flickers green, capable of poisoning enemies when it does.

Ipecac/ Scorpio: Overrides The Common Cold.
(Added in Afterbirth) Serpent's Kiss: Both poison effects work, but poison caused by Common Cold will not cause black heart drops. There is no way to tell which poison is applied.
(Pinky Eye) Pinky Eye: Activation rates are individually applied to each tear, effectively allowing for higher amounts of poison tears. However at 12 Luck, the trinket will not do anything for Isaac as The Common Cold is guaranteed to make every tear poisoned at that point.