Gigantifies Isaac for 30 seconds, persisting between rooms. While Gigantic, Isaac gains the following effects;
Gains a 4x damage multiplier and ~+7 range, but a 2.5x tear delay multiplier.
Becomes invulnerable to damage.
Can destroy obstacles and objects and open doors by walking into them. This includes metal blocks.
Deals extremely heavy contact damage to enemies (80 per tick, or 1,600 per second).
This item belongs to the Fun Guy set. Collecting three items from this set will transform Isaac into a walking mushroom.
Using Mega Mush again while its effects are still active will add an additional 30 seconds to the current active time.
Isaac's physical size, i.e. collision circle or "hitbox" is roughly doubled in radius when Mega Mush is active. This is the only item that can make Isaac tangibly larger than his starting size, as normal size ups can't increase the collision area beyond the default.
Mega Mush will not negate the damage taken from the Mausoleum door. It's even possible to die this way, while the item is active.
4.5 Volt: The contact damage dealt by Mega Mush will allow you to recharge it and use it multiple times in the same room.
Car Battery: Mega Mush's active time will be doubled to 60 seconds.
Mega Mush is based off of the Mega Mushroom from the New Super Mario Bros. series, which would turn Mario giant, grant invulnerability, and allow him to defeat almost anything he touches.
Originally in Antibirth, the item didn't gain charges while active, and was unlocked by defeating Mother (then known as The Witness) as every character on Hard Mode.
PC 04AW R3W6 (Treasure room adjacent to spawn)
PC FQZF GB13 (Treasure room adjacent to spawn)
Activating Mega Mush before entering the Knife Piece 2 area will keep the effect active inside the chase area but Isaac will have his normal sized, followed with a huge shadow and will continuously stomp the ground, breaking everything in his way, even pushing away Mother's Shadow.
Using Mega Mush in combination with Suplex! makes you grab enemies and move them around until you leave the room. Doing this on any of the Ultra Harbingers or The Beast will make the enemy disappear and render you unable to move at all.
Trying to leave the floor while Mega Mush is active and having any sort of item that grants flight or the character being able to fly by himself will freeze the game and prevent you from doing anything at all. You won't even be able to exit the run because the pause menu will not open. The game will still be responding and the only thing you can do is open up the debug console (if enabled). After Mega Mush's effect expires, the character will have the crying face they make when returning from Mega Mush's effect, making the game advance one frame, but you're still stuck until you forcefully exit the game or use the R key to restart.
Attempting this while in Tainted Jacob's lost form, and then proceding to the next floor after exiting the game and continuing the run will start you on the next floor still in lost form and without Dark Esau.