Isaac's tears are replaced by a knife that pierces enemies and can travel through obstacles. Holding down the fire button increases its range and power.
The knife deals damage 20 times per second upon contact with enemies while Isaac is holding it.
The knife deals 2× Isaac's damage while held and 6× damage at the furthest possible point of its throw distance, regardless of the actual maximum range stat.
Damage dealt by the knife depends on its charge state. The knife deals 2× Isaac's damage when held and 6× damage at about 1/3 of full charge. The knife damage quickly drops to 2× Isaac's damage when returning regardless of charge state. Further charging only increases its maximum range.
This item belongs to the Yes Mother? set. Collecting three items from this set will transform Isaac into his mother.
Apple! / Tough Love: The knife deals increased damage sometimes based on luck.
Brimstone / Sulfur: Shooting the knife also shoots a barrage of knives that goes across the screen, with each knife doing 80% ( 2×) of Isaac's damage; the extra knives do not return. The longer the charge, the more knives are added. This effect does not work with Azazel's default mini brimstone. The brimstone-knives do not benefit from A Lump of Coal.
Compound Fracture: The knife creates bone shards while in contact with an enemy.
C Section: Fetuses hold small knives that spin around them slowly, causing extra damage.
Diplopia / Crooked Penny: Each additional copy of the item will grant an extra knife that fires slightly diagonally from the original one, up to a maximum of 16. Picking up more than 16 knives won't fire more knives but will still modify the angle at which they're shot.
Epic Fetus: Each explosion shoots 10 knives in a circle from the point of impact.
Euthanasia / Little Horn: The knife can sometimes instantly kill enemies or deal extra damage to bosses.
Eye Sore: Will occasionally release 1-3 extra knives when shooting Mom's knife.
Flat Stone: Knife bounces as it flies away from Isaac, each bounce causing Isaac's tear damage.
Fruit Cake: The benefits of Fruit Cake still occur, but are short-lived due to the high damage of Mom's Knife.
The Forgotten: +4 damage. The knife will attach to The Forgotten's bone club, forming a scythe. Increases the size of both the melee swing and the bone throw, and allows a single melee swing to deal damage to an enemy twice instead of just once.
Ghost Pepper: The flames can shoot from Isaac while holding the fire button.
The flames only shoot when the fire button is released.
Holy Light: Holy Light can trigger on both held and thrown knives.
Ipecac: Knife turns green and poisons monsters that hit it. Removes Ipecac's damage increase.
Adds a flat +4 damage instead of Ipecac's +40. Knife turns green and poison monsters that hit it.
Adds a flat +2 damage and the knife explodes on contact, once per throw. Will not explode while held without throwing and while coming back to the Player after being thrown. Knife turns green and poison monsters that hit it.
Ipecac + Technology: Not recommended. Isaac will always take explosion damage when throwing the knife.
Invulnerability: Even though the knife cannot be fired, the knife will deal damage while Isaac is invincible.
Jacob's Ladder: The knife will sometimes generate sparks when hitting an enemy.
Lost Contact: The knife will block bullets, even when held (similar to Trinity Shield but has a smaller blocking area).
The Ludovico Technique: Isaac controls a floating knife that does 2× Isaac's damage. Tears stat is ignored and damage is dealt at a rate equivalent of 5 tear delay.
The Ludovico Technique + any multi-shot item: Isaac controls a floating knife surrounded by additional rotating knives.
Losing The Ludovico Technique will leave the additional knives, which are no longer controlled and appear in the same spot in every room.
The Ludovico Technique + Lost Contact: Floating knife loses almost all ability to block bullets, with a tiny area around the hilt that can stop shots.
The Ludovico Technique + Loki's Horns: Loses Loki's Horns effect; Isaac controls only one floating knife.
The Ludovico Technique + Brimstone: Mom's Knife overrides Brimstone, leaving only the floating knife.
Marked: The knife fires in the direction of the crosshair, charging automatically, with charging time proportionate to the distance of the crosshair from Isaac.
Mom's Eye: Another knife is permanently behind Isaac, able to deal contact damage. The chance to fire the rear knife is still affected by luck as usual.
Monstro's Lung: Firing a knife causes several other knives to shoot out in random directions.
The Parasite: The knife fires two knives in perpendicular directions from the first point of contact with an enemy. Additional knives can also fire more knives themselves.
The Parasite + Technology: Lasers will also split with the knives.
Pause: Mom's Knife will not stop the pause effect, allowing Isaac to kill or chip bosses easily by standing near them for the duration of the pause effect.
Proptosis: The Knife deals double damage regardless of range.
The Knife deals triple damage regardless of range.
Spirit Sword: Allows the spirit sword to be thrown during charged attacks.
Strange Attractor: Enemies are pulled towards the knife. The effect is minor while not firing and much stronger while the knife is launched.
Tainted Lilith: Her familiar will carry the knife. When firing once, Gello will not return back to Lilith, instead hanging around stationary. She can drag him around the room and the knife will still deal damage. Pressing any fire button again will whip out Gello, just as normal.
Technology Both the knife and the laser fire normally.
There is a connecting laser between Isaac and the knife, similar to The Forgotten's club throw.
Tammy's Head: Fires 10 knives in a circle around Isaac, doesn't add Tammy's Head's flat damage bonus.
Tiny Planet: The knife goes in a spiral around Isaac; the longer the knife is charged, the bigger the spiral. Not recommended, as this makes Mom's Knife almost impossible to aim.
Launched knife will orbit Isaac incredibly fast with a much larger hitbox, dealing consistent damage and covering every side of Isaac.
Items that fire knives out in a straight line, like Brimstone or Monstro's Lung, will cause the knives to circle around Isaac once, then shoot out in the intended direction in a straight line.
Homing tears cause the circling knife to abandon its orbit and behave more similar to how it works normally, the spinning knife will become purely visual as it deals no damage when the real knife finds a target to home into.
Tractor Beam: The knife will snap to whichever direction Isaac is firing in after it's thrown. However, it can no longer be fired diagonally.
In the original The Binding of Isaac, buying the item from the Devil Room cost 1 Red Heart container instead of 2.
In the original The Binding of Isaac, this item gave Isaac the appearance of his mother in her dress.
The description of the item is a possible reference to one of the Sniper's dialogue when performing the Huntsman taunt in Team Fortress 2.
Gallery[ | ]
A comparison between The Forgotten's bone attack with and without Mom's Knife.
Bugs[ | ]
Bug!
Tiny Planet and Homing tears may cause a visual bug when there is a high range. This makes it look like the knife is going in circles when it's not rotating around the body.
Bug!
With Tiny Planet, if Isaac releases the knife and exits the room before it returns to him, the knife will receive a significantly larger hitbox that scales with Isaac's Range. Releasing it and waiting until it returns will make it normal again.
Bug!
Using Mom's Knife and Marked with Haemolacria will cause the latter to only fire towards the center of room. Using the fire buttons will barely move the Marked crosshair, while Mom's Knife will be unusable and locked to the right.
Bug!
The Blindfolded condition also applies to the knives fired by Tammy's Head, and prevents it from working.
The knife deals 2× Isaac's damage while held and 6× damage at the furthest possible point of its throw distance, regardless of the actual maximum range stat.
Both the knife and the bombs fire normally.
Bug!
Tiny Planet and Homing tears may cause a visual bug when there is a high range. This makes it look like the knife is going in circles when it's not rotating around the body.
Using Mom's Knife and 
Marked with 
Haemolacria will cause the latter to only fire towards the center of room. Using the fire buttons will barely move the Marked crosshair, while Mom's Knife will be unusable and locked to the right.
The Blindfolded condition also applies to the knives fired by 
Tammy's Head, and prevents it from working.
Isaac Unlocks 
Satan Unlocks 
The Binding of Isaac: Rebirth
