1UP is a passive item.
A green mushroom with pale yellow spots.
Upon death, the player will respawn in the previous room with full HP, at the cost of using up the 1UP.
It is possible to find the 1UP multiple times in the same run, although this is rare.
- If you have collected 1UP and Ankh and die, only 1UP will trigger. The ankh will only trigger if you die a second time (provided you do not have another 1UP).
- If you collect 1UP while respawned as ??? using the Ankh, if you die you will respawn with no hearts shown on the screen and will only have one hearts worth of health before dying.
- If you have collected 1UP and Dead Cat, you will have 10 lives. Isaac, however, will always be reanimated with only 1 heart health in total, unless the 1UP was obtained afterwards.
- Sister Maggy or Little Steve will turn into Brother Bobby until exiting the room.
- Having a 1UP while doing a deal with the Devil that will kill Isaac, or dying at any other time when you have no normal Heart Containers, results in Isaac respawning with zero Heart Containers, unless Dead Cat was bought before.
- The 1UP's appearance and name is a reference to a Nintendo's popular franchise, Super Mario Bros. where a green, dotted mushroom would give the player an extra life if earned.
- Dying as you defeat a boss can provide you with both the item you earned and an opportunity to fight the boss a second time, earning a second item. This no longer works in v1.666.