Causes bombs to explode in a large cross-shaped blast by adding two additional explosions in each cardinal direction.
The damage for a single explosion is unchanged, but each successive explosion that hits the same enemy has its damage halved. The damage per successive explosion is: 60, 30, 15, 7.5, 3.75, 1.875, 0.9375, 0.46875, 0.234375
The damage for a single explosion is 0.75x times its normal damage, resulting in 75 damage per explosion. Each successive explosion that hits the same enemy has its damage halved. The damage per successive explosion is: 75, 37.5, 18.75, 9.375, 4.6875, 2.34375, 1.171875, 0.5859375, 0.29296875.
Mr. Mega changes the explosions' base damage to 110/185, with the same damage reductions as above.
Bombing the Donation Machine will cause a greater than normal amount of coins to drop from the Donation Machine.
The total amount of dropped coins is still deducted from the Donation Machine, however.
Bombing a Blue or Purple Fire Place may cause it to drop multiple Soul Hearts.
Bombs are colored blue and no longer have a skull on them.
Bob's Brain: Bomber Boy affects the explosion from Bob's Brain.
Dr. Fetus: Bomber Boy affects the bombs fired by Dr. Fetus.
Epic Fetus: Bomber Boy affects the missiles' explosions from Epic Fetus, with the explosions' multiplier resulting in 15 times Isaac's damage per explosion. Unlike with regular bombs, Bomber Boy won't decrease the damage from each of the missile's successive explosions, drastically increasing damage.
Mr. Mega: All explosions deal more damage, have a larger radius and will overlap each other more, potentially vastly increasing damage. Caution should be exercised, as the explosions' increased radius make self-damage likely.
Sad Bombs: Each explosion releases 10 tears in all directions.
Bomber Boy is a reference to the 1983 video-game Bomberman. Bomber Boy is, specifically, the original name of a Game Boy game known as Atomic Punk in the United States and Dynablaster in Europe and Oceania.
As of Repentance, the bomb's appearance when using this item also references how bombs look in Bomberman.
Before Repentance, the item's quote was "Explosive blast!".
PC YZLQ WLAV (Treasure Room adjacent to spawn on Normal, just to the left of spawn on Hard)
Due to the extreme force of multiple simultaneous explosions, it is possible for Bomber Boy bombs (especially when combined with Mr. Mega) to propel Isaac into obstacles or over pits.