On a prescription, the notation "prn" indicates that a medication is to be taken as needed instead of on a regular dosing schedule. The abbreviation is derived from the Latin "pro re nata," which translates to "as the thing is needed," according to the American Society of Health-System Pharmacists.
Medications only needed for a specific situation are sometimes only taken "prn." For example, medications that treat pain, constipation, allergy flares and even some types of chest pain are sometimes prescribed without a regular dosing schedule. When these medicines are prescribed, the prescribing doctor or pharmacist filing the prescription usually counsels the patient on when to take it, according to ASHP.