How to read Sig codes, Roman numerals and medical terms on a prescription
PTCB topic: Pharmacy calculations, Sig codes and abbreviations
- b.i.d., t.i.d. and q.i.d. mean 2, 3 and 4 times a day.
- q6h means every 6 hours, which is 4 doses a day.
- PO is by mouth, SL under the tongue, prn as needed.
- Roman numerals: a smaller one before a larger one is subtracted.
- Never copy QD, QOD, U, hs or AS onto a label; spell them out.
Key facts
- b.i.d. = twice a day, t.i.d. = three times a day, q.i.d. = four times a day, and q4h = every 4 hours.
- PO = by mouth, SL = under the tongue, PR = rectally, NPO = nothing by mouth, gtt = drops.
- a.c. = before meals, p.c. = after meals, prn = as needed, c̄ = with, s̄ = without.
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- Lower-case Roman numerals i, ii, iii and iv mean 1 to 4; a smaller numeral before a larger one is subtracted (XL = 40).
- ISMP lists QD, QOD, U, hs, ss, AD/AS/AU, OD/OS/OU and x3d as error-prone; the label uses words such as daily, units and left ear.
- Medical terms are built from word parts: nephr- = kidney, hepat- = liver, brady- = slow, -itis = inflammation, -ectomy = removal.
Quick check1 question · 2 more at the end
On a prescription, what does the Sig code b.i.d. mean?
Correct answer: D.
b.i.d. is short for the Latin bis in die, meaning twice a day. Its relatives are t.i.d. (three times a day) and q.i.d. (four times a day). A q followed by a number of hours, such as q2h, means every that many hours.
A Every 2 hours would be written q2h.
B Every other day is the error-prone QOD, which should be written out.
C At bedtime is the error-prone hs, which should be written out.
D Correct: b.i.d. comes from the Latin bis in die, twice a day.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Sig codes are shorthand for directions: b.i.d., t.i.d. and q.i.d. mean 2, 3 and 4 times a day, q6h means every 6 hours, PO means by mouth and prn means as needed. Lower-case Roman numerals give counts. Labels spell everything out, and error-prone codes such as QD, QOD, U, hs and AS are never copied.
1 What is a Sig, and why does it matter?The Sig is the prescriber's direction to the patient, written in shorthand. 1 min
The Sig is the prescriber's direction to the patient, written in shorthand. The word comes from the Latin signa, "write on the label." The 2026 PTCE outline lists Sig codes, Roman numerals, abbreviations, medical terminology and symbols in this area.
Your job is to turn the shorthand into plain words for the label, then use it for days' supply and quantity.
2 How often: which frequency codes do you need?Frequency codes tell you how many times a day, or how many hours apart, each dose is taken. 1 min
Frequency codes tell you how many times a day, or how many hours apart, each dose is taken. Most come from Latin.
| Code | Meaning | Doses a day |
|---|---|---|
| b.i.d. | twice a day (bis in die) | 2 |
| t.i.d. | three times a day (ter in die) | 3 |
| q.i.d. | four times a day | 4 |
| q4h, q6h, q8h | every 4, 6 or 8 hours | 6, 4, 3 |
| prn | as needed (pro re nata) | up to the limit |
A q followed by a number of hours means "every." So q6h is every 6 hours, which is 4 doses in 24 hours.
3 How and when: which route and timing codes appear most?Route and timing codes tell the patient where the drug goes and when to take it. 1 min
Route and timing codes tell the patient where the drug goes and when to take it.
- PO by mouth (per os); NPO nothing by mouth
- SL under the tongue; PR rectally
- IM into a muscle; IV into a vein
- gtt drops
- a.c. before meals; p.c. after meals
- c̄ with; s̄ without
Example: "1 tab PO q6h prn pain" becomes "Take 1 tablet by mouth every 6 hours as needed for pain."
4 How do Roman numerals work on prescriptions?Roman numerals add when values stay the same or get smaller, and subtract when a smaller one comes first. 1 min
Roman numerals add when values stay the same or get smaller, and subtract when a smaller one comes first. Prescribers often use lower-case i, ii, iii and iv for 1 to 4 tablets.
| I | V | X | L | C | D | M |
|---|---|---|---|---|---|---|
| 1 | 5 | 10 | 50 | 100 | 500 | 1,000 |
So XLV = (50 − 10) + 5 = 45, and "ii gtt" means 2 drops.
5 Which abbreviations should never reach the label?ISMP lists abbreviations that have caused harmful misreadings, and they should never be used. 1 min
ISMP lists abbreviations that have caused harmful misreadings, and they should never be used. You will still see some on handwritten prescriptions, so learn what was intended, then spell it out.
| Seen | Intended | Misread as | Label says |
|---|---|---|---|
| QD | every day | q.i.d. | daily |
| QOD | every other day | daily or q.i.d. | every other day |
| U | units | 0, 4 or cc | units |
| hs | at bedtime | half-strength | at bedtime |
| AD, AS, AU | right, left, each ear | OD, OS, OU (eyes) | right ear, left ear, each ear |
| ss | one-half | 55 | one-half (½) |
| x3d | for 3 days | 3 doses | for 3 days |
QD, QOD and U are also on The Joint Commission's "Do Not Use" list.
6 How do you decode medical terms?Medical terms are built from word parts, so you can often work out a new word. 1 min
Medical terms are built from word parts, so you can often work out a new word. Split it into prefix, root and suffix.
| Part | Meaning | Example |
|---|---|---|
| nephr- | kidney | nephrotoxicity |
| hepat- | liver | hepatotoxicity |
| cardi- | heart | bradycardia |
| brady- / tachy- | slow / fast | tachycardia |
| hypo- / hyper- | under / above | hypoglycemia |
| -itis | inflammation | dermatitis |
| -ectomy | surgical removal | cholecystectomy |
Common diagnosis abbreviations include HTN (hypertension), DM (diabetes mellitus) and GERD (gastroesophageal reflux disease).
7 What the technician doesThe technician translates the Sig and the pharmacist verifies it. 1 min
The technician translates the Sig and the pharmacist verifies it.
- Decode every part: dose, route, frequency, timing and duration.
- Write the label in plain words, never in codes.
- Use the frequency to calculate days' supply.
- Flag unclear or error-prone codes (ear versus eye, QD versus q.i.d.) for the pharmacist.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: q.i.d. means four times a day, not every day (QD).
Watch out: q6h is every 6 hours (4 doses a day), not 6 times a day.
Watch out: AS means left ear, not left eye (OS).
Watch out: Lower-case ii means 2, not 11.
Watch out: Write units as the word 'units', not U.
How to remember it
Read a Sig as "how much, which way, how often, how long."
Self-check:
- Translate "1 tab PO b.i.d. a.c."
- What is XLV, and what does ii gtt mean?
- Why should a label never say AS or QD?
Show answers
- "Take 1 tablet by mouth twice a day before meals."
- tab = tablet, PO = by mouth.
- b.i.d. = twice a day, a.c. = before meals.
- XLV = 45, and ii gtt = 2 drops.
- XL: the smaller X (10) comes before L (50), so subtract: 50 − 10 = 40.
- Add V: 40 + 5 = 45.
- ii = 2 and gtt = drops.
- Both are on ISMP's error-prone list.
- AS (left ear) can be misread as OS, an eye code.
- QD (every day) can be misread as q.i.d., four times a day.
- The label says "left ear" and "daily" instead.