A butcher's knife.
Isaac wears Mom's dress.
Isaac will have his tears replaced by a single, penetrating knife. It can be shot diagonally by using two arrow keys (holding up and right will aim it diagonally for example.) It can also be used as an upfront melee weapon, in which case it does high damage and possibly instantly kills any non-boss enemies that its edge touches. The knife deals three times the melee damage when thrown, then two times when it stops, and same damage as melee when it comes back. On the other hand, the DPS is higher when using the knife as a melee weapon. Familiars will fire tears while the knife is charging.
- The knife is tied to Isaac's base damage and shot effects (e.g. Spider's Bite).
- A high tears stat will shorten the charge duration for the knife.
- The distance the knife is able to travel is fixed at all times and therefore does not depend on the range stat. However, range ups will decrease the time it takes to charge the knife (similar to Tears).
- Brimstone, Dr Fetus - Mom's Knife overrides Brimstone and Dr Fetus with no special effect. From Rebirth onwards, Mom's knife with Brimstone will still be the same but, as the knife is fired, a wave of knives come shooting out of Isaac in a formation similar to that of Brimstone and Tractor Beam.
- If Dr Fetus has Sad Bombs combined with Mom's knife, then the bombs will shoot out 8 knives in a pattern similar to Tammy's Head.
- The Candle - If the Candle is activated while the knife is in mid-throw the knife will continue to move in a direction and speed that matches the movement of the player at the instant the candle was activated. The knife will continue to deal damage to whatever it touches and will return to the player as soon as the candle is used or deactivated. This also works with items such as Bob's Rotten Head or Notched Axe.
- IPECAC, The Inner Eye, Mutant Spider - Mom's Knife overrides all of these items, with a longer charge time and no damage increase. From Rebirth onwards, only The Inner Eye and Mutant Spider items will cause Mom's knife to have either three or four knifes fire at the same time.
- Technology - Mom's Knife overrides Technology with no special effect.
- Technology 2 - Mom's Knife overrides Technology 2, with the 65% damage still applying to Mom's Knife, making this synergy not recommended. Although the Technology 2 laser will still appear to fire, it is purely cosmetic and deals no damage.
- Mom's Eye - Mom's Knife completely overrides Mom's Eye if Mom's Eye is picked up after Mom's Knife
- Wiggle Worm - The knife stalls in the air when thrown upwards or downwards and wiggles up and down when fired left or right.
- Max's Head, Magic Mushroom - Mom's Knife does not stack with the 1.5x multipliers of these items or any other 1.5x multipliers.
- Certain tear affecting items also affect Mom's Knife, changing its color as well.
|The Common Cold|
Tints the blade green and gives it a chance to poison enemies.
Tints the blade yellow and makes it charge much quicker. The range down does not affect Mom's Knife in any way.
Tints the blade grey and increases damage.
|A Lump Of Coal|
Tints the blade black, but damage is unaffected.
Tints the knife red and provides a chance to temporarily freeze enemies.
Tints the blade purple and gives a slight homing effect.
Tints the blade grey and makes it spectral. This synergy has no real effect on Mom's Knife, as it can already go through obstacles. However, this does make destroying poop and fires with Mom's Knife more difficult.
The blade turns white and deals much more damage with shorter recharge time.
Turns the blade brown with no additional effects.
Tints the blade white and provides a higher chance of slowing enemies.
- My Little Unicorn/VII Chariot - In addition to the damaging on touch effect, the knife does not disappear, allowing you to stab, throw the knife, and charge around at the same time, dealing high amounts of damage.
- Pinky Eye - There is a 10% chance of the blade becoming poisonous (will appear green) upon entering a room.
- Polyphemus - Greatly increases the damage of Mom's Knife without any effect on charge time. This synergy is strong enough to kill most bosses in under a second.
- Mom's Bra - Isaac can deal high damage to any frozen enemy by simply standing near them, with little risk of being damaged himself.
- This item is based on the kitchen knife Mom took to sacrifice Isaac.
- Firing Mom's Knife will still leave small puddles as if Isaac were still firing tears.
- As shown by the video below, The Candle with Mom's Knife makes the knife fly slowly toward that direction until it reaches the end of the wall or until you use the Candle shot.
- Mom's Knife can sometimes float in the air aimlessly, even if it is not the candle glitch.
- If the player has 3 Dollar Bill, the knife will have a randomized color in every room. If the player has homing tears (purple knife) for example, the knife could be red, too.