Health

Isaac's health is shown at the top left of the screen, as a certain number of hearts.

The starting amount of health varies depending on the character. There are four different heart types: Red hearts, Soul/Spirit hearts, Eternal hearts, and Evil hearts.

Red Hearts
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This is the basic heart type. On floors prior to The Womb/Utero, taking damage from most things will result in losing half a heart. Explosions and spikes, however, will deal a full heart of damage. On any floor after The Womb/Utero, all forms of damage will deal a full heart, unless the player has [The Wafer], which will nullify this effect. Upon losing a red heart, a blank heart container will stay which can be filled by picking up more hearts.

Soul/Spirit Hearts
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Soul/Spirit hearts are blue and operate differently to red hearts. Unlike red hearts, these will disappear completley upon losing them. Therefore, they act as a sort of 'shield' to your red hearts. Damage works the same way.

Eternal Hearts
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Eternal hearts are quite rare and can only be found in halves. Instead of occupying a new space, they will replace half of another heart, turning that half white. They will disappear when you lose them. Finding two halfs of an eternal heart and completing it will act in the same way as a Health Up item/pill, and will give the player a new heart container. Completing a floor while still having half an eternal heart will do the same thing.

Evil Hearts
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More commonly known as black hearts, these are exactly the same as soul hearts, however they give the Necronomicon effect when losing them. However, it seems to do more damage than the average Necronomicon effect [citation needed].