Glossary
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.
The sig is the part of a prescription that says how to take the drug — the directions, as opposed to which drug it is. The word is short for the Latin signatura, and the shorthand inside a sig is a sig code. A prescriber writes 1 tab PO BID; the pharmacy turns that into the plain-English directions on the label, Take 1 tablet by mouth twice a day.
Nearly every sig code abbreviates a Latin phrase, which is why they resist being read as English. quaque (every) supplies the q in QD and Q4H; die (day) and hora (hour) supply the d and the h; bis, ter and quater (two, three and four times) supply BID, TID and QID; hora somni (hour of sleep) gives QHS; per os (by mouth) gives PO; and pro re nata (as the circumstance arises) gives PRN.
Three things about sig codes are on the exam and in the error literature. First, a code can be misread for a neighbor, which is why QD (daily) and QOD (every other day) sit on the Joint Commission's Official "Do Not Use" List and must be written out in full. Second, a code states a frequency, not a clock: BID is twice in a day, not exactly every 12 hours, and an order whose interval matters is written Q12H. Third, the label — not the sig — is what the patient follows, so the expansion has to be complete. A sig of 1 tab QID that reaches the patient as take as directed has lost the direction entirely.
Taught in
Related terms
- BID — BID means twice a day, from the Latin bis in die. It is two doses in a day rather than a strict every-12-hours schedule — the fixed-interval order is written Q12H.
- QID — QID means four times a day, from the Latin quater in die. It is the abbreviation most often misread as QD, and one tablet four times a day is the classic fourfold-overdose signature.
- 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.
- 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.
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)