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Unless it results in death, damage isn't permanent. Even death is reversible through powerful magic. Rest can restore a creature's hit points, and magical methods such as a cure wounds spell or a potion of healing can remove damage in an instant.
When a creature receives healing of any kind, hit points regained are added to its current hit points. A creature's hit points can't exceed its hit point maximum, so any hit points regained in excess of this number are lost. For example, a druid grants a ranger 8 hit points of healing. If the ranger has 14 current hit points and has a hit point maximum of 20, the ranger regains 6 hit points from the druid, not 8.