p.r.n.: Abbreviation meaning "when necessary" (from the Latin "pro re nata", for an occasion that has arisen, as circumstances require, as needed). One of a number of hallowed abbreviations of Latin terms that have traditionally been used in prescriptions.

Some others:

• a.c. = before meals (from "ante cibum", before meals)
• b.i.d. = twice a day (from "bis in die", twice a day)
• gtt. = drops (from "guttae", drops)
• p.c. = after meals (from "post cibum", after meals)
• p.o. = by mouth, orally (from "per os", by mouth)
• q.d. = once a day (from "quaque die", once a day)
• q.i.d. = four times a day (from "quater in die", 4 times a day)
• q._h.: If a medicine is to be taken every so-many hours (from "quaque", every and the "h" indicating the number of hours)
• q.h. = every hour
• q.2h. = every 2 hours
• q.3h. = every 3 hours
• q.4h. = every 4 hours
• t.i.d. = three times a day (from "ter in die", 3 times a day)
• ut dict. = as directed (from "ut dictum", as directed)