Dull Razor

Dull Razor is an unlockable activated item added in The Binding of Isaac: Afterbirth †.

Effect

Hurts Isaac without taking away health, giving him invincibility frames equal to a full heart damage and activating any items that trigger upon getting hit.

Notes

The item will cost 10 cents if found in a Shop
As using the Dull Razor will grant extra invincibility frames like normal damage, this item can be used to enter Curse Room for free, or, with proper timing, allow Isaac to avoid taking damage from an enemy or obstacle.
The invincibility frames allow Isaac to use Blood Donation Machines and Devil Beggars for free.
Dull Razor ignores shields granted by Holy Mantle, and similar items.

Synergies

Book of Virtues: Spawns a grey wisp on the middle ring. The wisp doesn't fire any tears. Whenever Isaac takes damage, there is a chance that the damage is negated and one of the wisps is destroyed instead. "Damage" taken from using Dull Razor does not count for this effect.
Car Battery: Dull Razor activates twice, triggering on-damage items twice. Invincibility frames are the same.
Cambion Conception: Dull Razor triggers a damage, allowing for free familiar spawn on repeated use.

Interactions

For a full list of items that work with Dull Razor, see the list of damage-triggered items. Note that any specific extra conditions the items may have for their activation still apply.

Blanket / Holy Mantle / Wooden Cross: Absorbs the "hit", wasting both items.
Car Battery: Dull Razor activates twice, but "damage" is only dealt once, not triggering synergies a second time.
Cone Head / Star of Bethlehem / Belly Jelly / Aura from White Poops or White Dips: Can block the "hit" Dull Razor inflicts, nullifying the invincibility frames and causing no effect.
Crown Of Light: Disables the effect for the current room.
Habit: Doesn't charge Dull Razor itself.
- Habit + Schoolbag: The secondary item will be charged.
  - Habit + Schoolbag + 9 Volt: If Dull Razor is copied into the second active item through an effect such as Diplopia or Crooked Penny, the two Dull Razors can be switched between infinitely.
- Habit + The Battery + Car Battery: Dull Razor will be charged, allowing for infinite uses of the item.
Isaac's Heart: Completely nullifies the effect of Dull Razor.
Perfection: Is not destroyed by a use of Dull Razor, as it counts as self-damage.
Purity: Cancels the current effect of Purity, allowing a different aura to be gained instead.