The chance to fire a razor blade depends on the luck stat, and the formula used to determine the chance is:
At base luck (0), the chance is 6.66%, maxing out at 100% at 14 luck.
Synergies
Cricket's Body / Compound Fracture / The Parasite: When razors are fired, the split tears will also be razors and do quadruple damage. The Parasite's split tears look like normal tears.
Dr. Fetus: Adds a chance for the bombs to deal increased damage. See the page for the specific damage formula.
Glass Cannon: Adds a chance to fire a giant razor that does roughly triple damage.
Adds a chance to fire a giant razor that does quadruple damage (x40 + 10 flat damage).
Haemolacria: Adds a chance for tears to do quadruple damage. Burst tears will also do quadruple damage if the main tear does quadruple damage.
Technology Zero: When razors are fired, they will connect to other tears via electricity. The electricity also does quadruple damage.
Lachryphagy: Tears can be fed to razors, increasing damage and splitting into 8 razors after a little while. If razors are fed to the tears, the razor damage doesn't apply to the tear both before and after splitting.
Interactions
Brimstone: Each entire beam has a chance to gain the damage boost.
Each tick of the beam has a chance to gain the damage boost.
Each tick of the laser rings have a chance to gain the damage boost.
Tough Love: With 14 luck, overrides Tough Love, as the razor blades do more damage anyways. With 13 luck, Apple and Tough Love each have about an equal chance to trigger.
Both the item itself and the description is a reflection of one of Isaac's nightmares between floors that involves him finding a razor blade inside a piece of candy.
The item and pickup quote are a reference to the urban legend of giving away candies with razor blades hidden inside of them as a way of harming people.
Cricket's Body / 
Compound Fracture / 
The Parasite: When razors are fired, the split tears will also be razors and do quadruple damage. The Parasite's split tears look like normal tears.
Dr. Fetus: Adds a chance for the bombs to deal increased damage. See the page for the specific damage formula.
Glass Cannon: Adds a chance to fire a giant razor that does roughly triple damage.
Adds a chance to fire a giant razor that does quadruple damage (x40 + 10 flat damage).
Haemolacria: Adds a chance for tears to do quadruple damage. Burst tears will also do quadruple damage if the main tear does quadruple damage.
Technology Zero: When razors are fired, they will connect to other tears via electricity. The electricity also does quadruple damage.
Lachryphagy: Tears can be fed to razors, increasing damage and splitting into 8 razors after a little while. If razors are fed to the tears, the razor damage doesn't apply to the tear both before and after splitting.
Brimstone: Each entire beam has a chance to gain the damage boost.
Tech X: Overrides Apple!.
Tough Love: With 14 luck, overrides Tough Love, as the razor blades do more damage anyways. With 13 luck, Apple and Tough Love each have about an equal chance to trigger.
The Binding of Isaac: Rebirth
