By means of a neutralize poison spell, the priest detoxifies any sort of venom in the creature or substance touched. Note that an opponent, such as a poisonous reptile or snake (or even an envenomed weapon of an opponent) unwilling to be so touched requires the priest to roll a successful attack in combat. This spell can prevent death in a poisoned creature if cast before death occurs. The effects of the spell are permanent only with respect to poison existing in the touched creature at the time of the touch; thus, creatures (and objects) that generate new poison are not permanently detoxified.
The reversed spell, poison, likewise requires a successful attack roll, and the victim is allowed a saving throw vs. poison. If the latter is unsuccessful, the victim is incapacitated and dies in one turn unless the poison is magically neutralized or slowed.
Notes: Common for clerics and druids (PHB).