Razor Blade 
Effects
Notes
- (Added in Afterbirth) (Eve) Eve starts with this item after unlocking it and donating 439 coins to the Greed Machine.
- (Removed in Repentance) 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.
- (Added in Repentance) Eve now starts with the Razor Blade if unlocked from the Greed Machine, even if Satan has not been defeated with Eve.
- (Removed in Repentance) 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.
- If Isaac doesn't have a full Red Heart or a filled Bone Heart, then Soul, Black, and empty Bone Hearts are taken instead.
- The effect stacks with itself and (Red Patch) Red Patch, which provides a similar benefit.
- Using this item does not affect Devil or Angel room chances.
- Using this item as (The Lost) The Lost will result in death.
- When picked up, Isaac's tears turn red, though this change is purely cosmetic.
- The Razor Blade prioritizes Red Hearts first.
- Using the Razor blade grants Isaac the regular immunity frames upon usage; this allows Isaac to enter cursed rooms without using soul hearts should he have them.
- Using this item in the Greed mode Ultra Greed boss room will result in Isaac losing 2-4 coins and dropping 1-3 pennies on the floor similar to being hit by any of the greed enemies.
Synergies
- (Added in Repentance)
Book of Virtues: Spawns red wisps in the middle ring. Wisps have higher health. - (Removed in Repentance)
Isaac's Heart: Isaac cannot take damage from the Razor Blade, allowing for infinite uses of the item. 
Old Bandage / (Added in Afterbirth †)
Shard of Glass: Isaac has a chance to drop a red heart while using the item.- (Added in Repentance)(Panic Button) Panic Button: Self-damage occurs before enemy damage, making it almost impossible to lose Angel or Devil chance.
- (Removed in Repentance)
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: The Razor Blade can be used to activate this item's effect by doing enough damage to get Isaac down to half a red heart (or one red heart as Eve), providing a big damage boost for the current room at the cost of health.- (Added in Repentance)
Birthright & (Judas) Judas: Actives the effect of 
Blood Rights alongside it's normal effect.
Interactions
- (Added in Afterbirth)
Car Battery: No effect. - (Added in Repentance) Shield/Invulnerability: Will not protect against this item's damage.
- (Added in Repentance) (Tainted Samson) Tainted Samson: Using this item can make Tainted Samson unkillable, as his Berserk timer gains time when damaged, and he can't die from lack of red hearts during the rage.
Bugs
| (Bug) 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. |