Dull Razor ignores the shield, giving its normal effect without wasting the shield.
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, but "damage" is only dealt once, not triggering synergies a second time.
Dull Razor activates twice, triggering synergies a second time.
Cone Head: Can block the "hit" Dull Razor inflicts, causing no effect.