(in prescriptions) as the occasion arises; as needed.
Origin of p.r.n.
From the Latin word prō rē nāta
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
pro re nata (as the situation demands)
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.