(medical abbreviation)
PRN is an abbreviation for the Latin phrase pro re nata meaning "according to need". PRN is used in the signature line of written prescriptions, and basically means to take a medication when it is needed - often for pain medicines.
Pro re nata when used as a legal term takes on a slightly different meaning of "for the existing occasion" or "as matters are".
Author's note: I cannot find any information that validates the definition in a previous writeup within this node stating that PRN stands for "per requisitam necessitatem"