Incubus: Creates additional Incubus familiars, which share the same tear modifiers as the player.
Multidimensional Baby: Each copy of Multidimensional Baby follows the previous copy with a 2.5-second delay.
Smart Fly: Very useful on Greed mode, as using it will add an attacking fly, killing enemies and bosses very quickly.
Twisted Pair: Creates additional Twisted Pairs with the same tear modifiers as the player. Multiple twisted pairs will line up horizontally, covering a broader area.
1up! / Dead Cat: Familiars are multiplied but do not give Isaac additional lives.
Abyss: Flies gained from Abyss do not count as familiars for Box of Friends, and are not copied.
Best Bud: If used before taking damage, the white fly will circle Isaac. However, multiple white flies cannot be spawned, so using Box of Friends after taking damage or using it multiple times in a room creates no effect.
Book of Virtues: Spawns black wisps on the middle ring. After creating a wisp, Box of Friends also gives temporary copies of all of Isaac's wisps, including any extra copies Box of Friends made.
Buddy in a Box: Creates another randomly generated familiar, rather than copying the same one.
Bumbo: Will spawn a level 1 Bumbo regardless of the original Bumbo's current level.
Milk!: The familiar itself doesn't appear, but instead, it decreases tear delay by 2 (effective tears up) when used, without needing to take damage.
No extra tear delay is granted, neither before or after breaking.
Monster Manual: Familiars spawned by Monster Manual are not copied. It will still spawn a Demon Baby if a familiar from Monster Manual is generated and there are no other valid familiars that can be duplicated.