Both conditions must be met for Eve to hold it; donating 439 coins before defeating Satan with Eve will wrongly state on Eve's profile on the Character Selection Screen that she has it equipped.
Eve starts with the Razor Blade if unlocked from the Greed Machine, even if Satan has not been defeated with Eve.
If Isaac has half a red heart left, Razor Blade will damage other heart types.
It is possible to die using Razor Blade if Isaac does not have enough hearts.
The effect stacks with itself and Red Patch, which provides a similar benefit.
Using the Razor Blade grants immunity frames; this allows access to Curse Rooms without losing soul hearts.
Using this item in the Ultra Greed boss room will remove 2-4 coins and drop 1-3 pennies on the floor, as if Isaac was hit by an enemy attack.
Using this item as The Lost will kill The Lost, even if the protective effect from Holy Mantle is still active.
This interaction will kill Isaac while temporarily transformed into The Lost by a White Fire Place, even when he would otherwise have enough hearts to survive.
Synergies
Invulnerability / Shield: The Razor can be used at no cost while invulnerability / shield is active, allowing damage to be significantly increased for the room.
Tainted Samson / Berserk!: The Razor Blade can be used in order to gain a massive damage boost in a room as long as there's a source of HP within reach.
The item itself is a reference to the act of cutting one's self, usually at the wrists, to inflict self-harm or to commit suicide by loss of blood.
Isaac's appearance after using the item is just like the Dople.
In the original The Binding of Isaac, Razor Blade was unlocked by defeating Mom's Heart / It Lives! as Eve.
Bugs
Bug!
As Eve, there is a very unlikely chance that using this to activate Whore of Babylon will not activate it, despite being at the correct health to do so.
Red Hearts before other heart types.
When used more than once in the same room, damage taken is lowered to half a heart.
Eve starts with this item after unlocking it and donating 439 coins to the 
Greed Donation Machine.
Both conditions must be met for Eve to hold it; donating 439 coins before defeating 
Satan with Eve will wrongly state on Eve's profile on the Character Selection Screen that she has it equipped.
Red Patch, which provides a similar benefit.
Devil Room/
Angel Room chances.
Curse Rooms without losing soul hearts.
Ultra Greed boss room will remove 2-4 coins and drop 1-3 pennies on the floor, as if Isaac was hit by an enemy attack.
The Lost will kill The Lost, even if the protective effect from 
Holy Mantle is still active.
White Fire Place, even when he would otherwise have enough hearts to survive.
The Book of Belial + 
Birthright as 
Judas / 
Dark Judas: Invokes the effect of 
The Necronomicon effect along with the original effect.
Book of Virtues: Spawns red wisps in the middle ring. Wisps have higher health.
Isaac's Heart: Isaac cannot take damage from the Razor Blade, allowing for infinite uses of the item.
Old Bandage / 
Shard of Glass: Isaac has a chance to drop a red heart while using the item.
Panic Button: Self-damage occurs before enemy damage, making it almost impossible to lose Angel or Devil chance.
The Polaroid: Isaac takes no damage while invincible, allowing for repeated free uses of the item.
The Wafer: Isaac loses half a heart instead of a full heart.
Whore of Babylon/
Dark Prince's Crown: Can lower Isaac's health on demand to activate the bonus.
Car Battery: No effect.
Tainted Samson / 
Berserk!: The Razor Blade can be used in order to gain a massive damage boost in a room as long as there's a source of HP within reach.
The icon of Razor Blade as seen on the lower 3DS screen.
Dople.
Mom's Heart / 
It Lives! as 
Bug!
Eve Unlocks 
Satan Unlocks 
Greed Donation Machine Unlocks 
The Binding of Isaac: Rebirth
