Upon activation, Isaac will turn black with twisted horns, gain 1 soul heart, gain increased Damage of 0.7 and have his Speed decreased by 0.18 for the current room; allows Isaac to break rocks and damage enemies by simply walking over them (while active).
- As with most Activated Items, Items such as Habit, 9 Volt and The Battery decrease the charge time, increasing the effectiveness of this item.
- With both the 9 Volt and The Battery, The Nail can be triggered once for every room containing an enemy, provided the player waits for The Battery to activate at least twice before clearing the room. Because there is no upper limit on the number of Soul Hearts a player can carry, this can effectively add an additional Soul Heart pickup to every enemy-containing room.
- Having both Habit and The Wafer with this item will make the player invincible, as for every two hits (always only half a heart due to The Wafer) the player takes in a room will fully charge this item, allowing it to be used to regain the lost health in the form of a Soul Heart.
- Much like The Book of Revelations, The Nail works particularly well with the XRay Vision-Secret Room-The Battery exploit, potentially providing a patient player with a near-infinite number of Soul Hearts.
- The Virus' contact damage overrides The Nail's contact damage effect, dealing less damage.
- Isaac's appearance only changes upon activation (until the room is left), simply picking the item up won't change his appearance.
- Using this item causes Isaac to produce a demonic grunt when hit, as opposed to the usual whimpering sound.
- Unlocked by beating Mom's Heart 3 times.
- The Nail is most likely a reference to the nails used to pin Jesus to the cross. These nails are said to have wondrous effects.
- The Nail's effect could be reference to Castlevania 2 : Simon's Quest for the NES, where one of the items needed to progress is Dracula's Nail, which allows the character to break blocks when it is equipped.
- The Nail, like all other contact damage items, cannot inflict contact damage to Hearts or Mom's Heart/It Lives, but can do so to other enemies that don't deal contact damage to the player.