Glossary
PRN
PRN means "as needed", from the Latin pro re nata. By itself it sets no schedule and no ceiling, so the sig must still state the dose, the shortest interval allowed, and the maximum in 24 hours.
PRN is from the Latin pro re nata, "as the circumstance arises", and it is the abbreviation every pharmacy writes for as needed. 1 tab PO PRN pain means one tablet by mouth when the patient has pain.
PRN is worth a page because of what it does not say. Unlike BID or Q4H it sets no clock, so three facts have to come from the rest of the sig or the prescriber's directions: the amount per dose, the shortest interval allowed (Q4H PRN), and the ceiling in 24 hours. A PRN order with no ceiling is a live source of harm, which is why the safety literature treats it as an incomplete order rather than a permissive one.
The exam asks PRN two ways. As a translation it is simply as needed. As a calculation it is always tested with a stated limit — "PRN nausea, up to 4 doses a day" — and the days supply or the maximum daily dose is computed from that limit, never from the number of doses the patient might take. PRN also carries a labeling duty: the pharmacy prints the indication the prescriber gave, because a PRN drug with no stated purpose is one a patient will take for the wrong symptom.
Taught in
Related terms
- sig code — The sig is a prescription's directions, and a sig code is the shorthand written inside it — BID for twice a day, QHS for at bedtime. The pharmacy expands each code into plain English on the label.
- QD — QD means once a day, from the Latin quaque die. It is on the Joint Commission's Do-Not-Use List because QD and QID are misread for each other, so the order is written out as "daily".
- QHS — QHS means at bedtime, from the Latin hora somni — the hour of sleep. ISMP lists it as error-prone because HS also reads as "half strength", so the words are written out instead.
Source: PTCB — PTCE Content Outline, effective January 6, 2026 (Domain 4 / KA 4.1, "Interpret prescription and medication orders … sig codes, abbreviations, medical terminology and symbols, and days supply", quoted verbatim); The Joint Commission, Official "Do Not Use" List (NPSG.03.05.01 — the minimum list an accredited organization must adopt, read in full); ISMP, List of Error-Prone Abbreviations, Symbols, and Dose Designations (the labels, symbols and abbreviations ISMP asks prescribers to write out) (outline accessed 2026-10-03, verbatim; the Joint Commission and ISMP lists read from their own fact-sheet and recommendation pages 2026-10-06 — each is revised on its own schedule; the definitions here restate this course's own order-interpretation-and-sig guide, which is the page whose sources back every definition above)