Temporary Hit Points (PSR Supplement)
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Some effects confer temporary hit points (THP) to a creature. These aren't actual hit points; they're a buffer against damage.
When you have THP and take damage, your THP are lost first. Any leftover damage carries over to your normal hit points. If for example you have 5 THP and then take 7 damage, you lose all your THP, and then take 2 damage.
Because THP are separate from your actual hit points, they can exceed your hit point maximum. You can receive THP even while at your hit point maximum.
If you are at 0 hit points, receiving THP doesn't bring you back to consciousness, but they can still absorb damage and potentially prevent your death.
Any THP you have last until they're depleted, until you camp, or until the granting effect ends.