Grants a 15% chance to fire poison tears, each tick dealing damage equal to Isaac's.
Poison from the shots will tick 2 times.
Inflicts poison on enemies who make contact with Isaac.
Contact poison will tick 6 times.
Enemies who are killed after they have been inflicted with this type of poison have a 20% chance to drop Black Hearts. Enemies can still drop black hearts after the poison has worn off.
Enemies that make contact with Isaac have a 20% chance to drop a Black Heart. Enemies can still drop black hearts after the poison has worn off.
Enemies and bosses with multiple segments, such as Chub and Larry Jr., have a chance to drop multiple Black Hearts.
If a boss would have been poisoned, but negates it with status immunity, they can still drop a black heart.
Contact poisoning enemies that don't deal contact damage can be an easy way to generate black hearts (e.g. all variants of Pooter, Host, Boil, Cod Worm, and any of Ultra Greed's non-blank coins).
Although Stonies and Shopkeepers can't be visibly poisoned, they still can drop Black Hearts if destroyed after being touched.
Synergies
Dr. Fetus / Epic Fetus: Bombs and missiles may become poisonous, and enemies hit by them may drop black hearts.
Bombs no longer grant black hearts but poison bombs leave a poison cloud similar to Bob's Curse.
Gnawed Leaf: Standing still allows for safe kills and possibly spawn infinite black hearts.
Mars: Dashing into enemies will often both kill enemies, and cause them to drop black hearts.
Mom's Knife: Enemies poisoned by the knife may drop black hearts.
Tainted Forgotten: Especially strong as you are likely to apply contact poison when attacking enemies, giving you black hearts frequently.
Any item or card that grants Isaac temporary invincibility lets Isaac use Serpant's Kiss on any enemy without taking damage. This includes, but is not limited to:
Serpent's Kiss is most likely referencing Viper (Madame Hydra) from Marvel Comics, who has green lipstick and has transmitted poison through hollow fangs or poisoned lipstick before.
Enemies who are killed after they have been inflicted with this type of poison have a 20% chance to drop 
Black Hearts. Enemies can still drop black hearts after the poison has worn off.
Enemies that make contact with Isaac have a 20% chance to drop a 
Pooter, 
Host, 
Boil, 
Cod Worm, and any of 
Ultra Greed's non-blank coins).
Stonies and 
Shopkeepers can't be visibly poisoned, they still can drop Black Hearts if destroyed after being touched.
Dr. Fetus / 
Epic Fetus: Bombs and missiles may become poisonous, and enemies hit by them may drop black hearts.
Bob's Curse.
Gnawed Leaf: Standing still allows for safe kills and possibly spawn infinite black hearts.
Mars: Dashing into enemies will often both kill enemies, and cause them to drop black hearts.
Mom's Knife: Enemies poisoned by the knife may drop black hearts.
Tainted Forgotten: Especially strong as you are likely to apply contact poison when attacking enemies, giving you black hearts frequently.
VII - The Chariot/
VII - The Chariot?
A Pony/
White Pony
Book of Shadows/
Celtic Cross/
Empty Vessel/
Rune of Algiz/
The Polaroid/
Virgo
Breath of Life
Dull Razor
Holy Mantle
My Little Unicorn/
Unicorn Stump/Feels like I'm walking on Sunshine!
Aries (if walking speed is at least 0.85)
The Gamekid/Power Pill!
Ventricle Razor (briefly after exiting a portal)
Godhead: The Godhead aura cannot inflict poison but the tears themselves may poison enemies on contact.
Athame / 
Maw Of The Void: The black rings may poison enemies but do not generate black hearts.
Brimstone: The blood barrage may poison enemies but does not generate black hearts.
Tech.5 / 
Tech X / 
Technology / 
Technology 2: The lasers may poison enemies but do not generate black hearts.
Trisagion: May poison enemies but does not generate black hearts.
Lilith Unlocks 
Satan Unlocks 
The Binding of Isaac: Rebirth
