Glossary
PO
PO means by mouth, from the Latin per os. It is the route written in the sig — 1 tab PO BID — and ISMP lists it as error-prone because the shorthand is not readable to every patient.
PO means by mouth, from the Latin per os. It is the route in a sig: 1 tab PO BID is one tablet by mouth twice a day, and it tells the technician the dose is swallowed rather than applied, injected or placed under the tongue.
PO is the route the error-prone abbreviation lists single out. ISMP's List of Error-Prone Abbreviations includes per os (PO) and asks for by mouth instead, for the plain reason that a patient reading PO on a label may not know it means anything, while by mouth cannot be misread. The rest of the routes carry the same risk in different spellings: SL (sublingual), PR (rectal), IM, IV and subcut/SC, top (topical), and the eye and ear abbreviations — OD, OS, OU, AD, AS, AU — which the Joint Commission bans outright in favor of right eye, left eye, both eyes, right ear, left ear, both ears.
Two traps sit beside PO. NPO (nil per os) is not a route to give a drug by — it means nothing by mouth, and it is a holding order, not a sig. And a PO order is only fillable if the drug has an oral form: an order that reads PO for a drug that exists only as an injection is a prescribing error, and catching it is the technician's job at data entry.
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.
- 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.
- 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".
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)