BMS 113 · Common Core (Tronc commun)Section 6.2 · Prescription writing, dosage and safe medicine use
Section 27 of 31
Prescription abbreviations and measures
Interpret the common prescription abbreviations and household measures, and explain why ambiguous abbreviations are dangerous.
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Why this matters
An abbreviation that can be read in two ways turns a safe order into a dangerous one, and many medication errors begin with a symbol rather than with a drug. If you cannot expand qid, 3/7 or a tablespoon correctly, you may give four times too much, stop a treatment too early or measure the wrong volume. You will use this reading skill again in Unit 6.3, where you calculate quantities from the same orders, and in Unit 6.5, where misread orders become patient harm.
What you need to know first
You only need the everyday idea of a short recipe note and the fact that a day has 24 hours. Unit 6.1 shows you where these abbreviations sit among the three parts of a prescription; this section teaches you to read them safely.
Words to know (6)
Abbreviation
A shortened form of a word or phrase. It is safe only while every reader expands it in exactly the same way. “bid” saves three words but can be misread as a similar symbol.
Frequency abbreviations (qd, bid, tid, qid)
Short forms telling how often a dose is repeated: qd once a day, bid twice, tid three times and qid four times a day. “1 tab qid” means four tablets in one day.
Route abbreviations (po, per os)
Short forms telling how the drug enters the body; po and per os both mean by mouth, while Per means through or by. A syrup written “po” is swallowed, not rubbed on the skin.
Duration notation (3/7, 7/7, 1/52)
A way of writing how long treatment lasts: the number before the slash counts days, and 7 means days, 52 weeks and 12 months. 7/7 is one week and 3/52 is three weeks.
Household measure
An approximate home volume: one teaspoon is 5 ml, one dessertspoon 10 ml, one tablespoon 15 ml, one glass 240 ml and one dropper 0.06 ml. “1 tablespoon bid” means 15 ml twice a day.
Trailing zero
A zero written after a decimal point, as in 5.0 mg. It is avoided because a lost point can turn it into 50 mg. Write 5 mg rather than 5.0 mg.
One idea at a timeIdea 1 / 6
In one sentence Prescription abbreviations save space, but they are safe only when every reader expands them in exactly the same way.
Abbreviations save space and create risk
Prescriptions have traditionally been written in a compressed mixture of Latin and English abbreviations. They save time, fit more information on a small sheet, and are understood by pharmacists trained in the same convention. The same compression creates risk. An abbreviation is safe only while every reader expands it in the same way; the moment one reader sees a frequency where another sees a total, or milligrams where another sees micrograms, the order changes meaning without anyone noticing. The lecture notes therefore present the list together with the modern advice to prefer unambiguous full words wherever confusion is possible. Learning the list is a reading skill, not a licence to use every item on it.
For example
“qd” and “qid” differ by one letter, but one means once a day and the other four times a day — a fourfold difference in exposure.
Think of it like
An abbreviation is like a shortcut that only works if everyone takes the same path, and unlike a shortcut on a map it has no sign warning you when you have taken the wrong one.
Pause and check yourself
When is an abbreviation safe to use on an order?
Show the answer
Only while every reader expands it identically. As soon as two readers understand it differently, the order changes meaning without anyone noticing.
Read detailed explanations
Where the traditional abbreviations came from, and why they persist
Most of the traditional list descends from Latin prescriptions, which is why the same symbols recur across countries and languages. Rx is the familiar opening for “take thou”; b.i.d., t.i.d. and q.i.d. abbreviate the Latin for twice, three times and four times a day; p.o. stands for per os, “by mouth”; p.c. stands for post cibum, “after meals”; h.s. stands for hora somni, “at the hour of sleep”; and gtt stands for guttae, “drops”. A shared vocabulary was genuinely useful when prescriptions crossed language borders and when a small sheet had to carry a complete order. It persists for three reasons: habit, the speed it gives an experienced writer, and the absence of a standard alternative in some settings. None of these reasons outweighs safety when an abbreviation is ambiguous, which is why the modern advice is not to abandon the list but to narrow its use. Understanding the origin also helps a beginner predict where confusion will arise: Latin pairs such as q.d. and q.i.d., or u.d. and u.i.d., look alike on a handwritten line, and the logic of the language does not protect the reader. The useful skill is therefore selective: read the whole list fluently, and write almost none of it.
The anatomy of a dangerous abbreviation
A dangerous abbreviation has a recognisable anatomy. First, it is short — usually two or three characters — so a single misread letter changes most of its meaning. Second, it belongs to a family of similar-looking symbols, such as qd, qid, qhs and qh, or OD and qd, so the reader’s expectation, not the ink, decides what is seen. Third, it encodes a number or a unit, which is where the smallest writing error becomes the largest clinical error: “mg” and “µg” differ by one small stroke but by a factor of one thousand. Fourth, it is often placed close to a decimal point or a slash, two marks that are easily lost in handwriting: 0.5 becomes 5, and 3/7 becomes 37. Finally, it usually appears in a rushed, interrupted workflow where the reader completes the expected word rather than the written one. Each of these features suggests a defence. Write the meaning in full, separate similar products on the shelf, use tall-man lettering to make look-alike names visibly different, read back a verbal order, and have a second person check a high-risk calculation. None of these defences relies on the reader being more careful; each changes the system that produces the error.
Explore the diagram
Look at the whole drawing first. Then select one numbered part and connect it to the idea you just read.
Prescription abbreviations and household measures
A two-column reference card: prescription abbreviations with their meanings and duration notation on the left, household measures with volumes and a safety caution on the right.
New to diagrams? Read this first
Look at the whole drawing once without clicking anything. Ask yourself only: what kind of picture is this (a comparison, a route, a sequence, a table)?
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01 / 4Abbreviations block
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Abbreviations block
Prescription abbreviations are a shared shorthand between prescriber and dispenser: Aa means of each, C means with, Caps means capsules and Gtt means a drop. They save time on a busy ward, but each one must be understood exactly, because a misread abbreviation becomes a wrong dose.
Schematic; not to scale. The table reproduces the abbreviations and household measures given in the lecture notes; in practice a prescriber should spell out any abbreviation a reader might misread.
Key points to remember
✓Abbreviations save space but are safe only when every reader expands them identically.
✓Route and timing abbreviations such as po and pc describe how and when, not how much.
✓Frequency and duration abbreviations such as qid and 3/7 fix the schedule and the length of treatment.
✓Writing full words, full units and a leading zero removes most opportunities for a misreading.
Go deeper
Worked example
Translating an order into plain language. A prescription reads: “Amoxicillin 125 mg/5 ml syrup — 1 tablespoon po bid pc × 5/7 — shake before drinking.” A parent asks the pharmacist exactly what to do. The pharmacist must expand each abbreviation without guessing.
“po” means by mouth: the syrup is swallowed, not applied to the skin and not injected.
“bid” means two times daily, so the child takes one tablespoon in the morning and one in the evening.
“pc” means after meals: each dose follows food, which is part of the order rather than a suggestion.
“× 5/7” means for five days: the number before the slash counts the days, in the same pattern as 3/7 for three days and 7/7 for one week.
“1 tablespoon” is a household measure of 15 ml, not a teaspoon of 5 ml; and “shake before drinking” keeps the active ingredient evenly dispersed in the suspension so that each dose is the same strength.
Each symbol is a separate instruction. Expanding them one at a time, and never assuming, is what turns an abbreviation list into a safe dose.
Common misconception
Beginners often treat the abbreviation list as interchangeable shorthand and assume that readers will “know what was meant”. Abbreviations are safe only when everyone expands them identically; the list is a reading skill, and the safest modern order uses full words.
Check your reasoning
A prescription says “1 tab qid”. How many tablets is that in a day, and what would the patient take if it were misread as “qd”?
Reveal an answer
qid is four times a day, so four tablets; qd is once a day, so one. The same letters in a different order change the daily amount fourfold, which is exactly why full words are preferred.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Prescription abbreviations and household measures, pp. 54–56
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About this course and its sources
Bilingual lessons organised from the supplied BMS 113 General Pharmacology I lecture notes. Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.