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.
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.
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.
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.
The item will cost 10 cents if found in a 
Shop
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.
Holy Mantle, and similar items.
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.
Cambion Conception: Dull Razor triggers a damage, allowing for free familiar spawn on repeated use.
Blanket / 
Wooden Cross: Absorbs the "hit", wasting both items.
Cone Head / 
Star of Bethlehem / Aura from 
White Poops or White Dips: Can block the "hit" Dull Razor inflicts, causing no effect.
Crown Of Light: Disables the effect for the current room.
Habit: Doesn't charge Dull Razor itself.
Schoolbag: The secondary item will be charged.
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.
The Battery + 
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.
Bug!
Eve Unlocks 
Delirium Unlocks 
The Binding of Isaac: Rebirth
