Lesson · 24 min

Reading Prescriptions and Pharmacy Terminology

The abbreviations, the parts of a prescription, and the ones that are no longer safe to use.

By the end of this lesson you can

Identify the required elements of a valid prescription
Interpret common sig abbreviations
Recognize the abbreviations on the do-not-use list and why
Convert a sig into plain-language patient instructions

📘 Reading Lesson

Lesson Notes

Read through the key concepts before you try the challenge.

The sig is where errors enter

On the job

You are entering a prescription.

The sig reads “1 tab PO QD.” You read it as QID and enter four times daily instead of once. Both are real abbreviations, they differ by one letter, and the patient takes four times the intended dose.

Your task: Learn the abbreviations, and learn which ones should no longer appear at all.

Element	What it is
Patient information	Name and date of birth, at minimum
Date written	Affects validity and refill timing
Inscription	The drug, strength, and quantity
Sig	The directions for the patient
Refills	How many, if any
Prescriber information	Name, signature, and identifiers
DAW	Dispense as written — whether substitution is allowed

Parts of a prescription

Abbreviation	Means
PO	By mouth
BID	Twice daily
TID	Three times daily
QID	Four times daily
PRN	As needed
AC / PC	Before meals / after meals
HS	At bedtime
gtt	Drop
SL	Sublingual, under the tongue
Disp	Dispense

Sig abbreviations you will see constantly

Several abbreviations appear on the Institute for Safe Medication Practices do-not-use list because they have a documented history of causing harm. U for units is read as a zero, turning 4U into 40. QD and QOD are mistaken for QID. A trailing zero — 1.0 mg — is read as 10 mg when the decimal point is faint, and a missing leading zero — .5 mg — is read as 5 mg. Write the word, use a leading zero, never use a trailing one. If a prescription arrives using them, that is a reason to verify rather than to interpret.

Worked example

Turning a sig into patient instructions

The sig reads: 1 tab PO BID PC × 10 days. Write what goes on the label.

1
Translate each element separately rather than reading it as a phrase.
1 tab is the dose, PO is the route, BID is the frequency, PC is the timing, 10 days is the duration. Translating piece by piece catches an element you would otherwise skip.
2
Write it in the order a patient acts on it.
“Take one tablet by mouth twice daily after meals for 10 days.” The patient needs how much, how, how often, when, and for how long — in that order, because that is the order they will do it in.
3
Avoid the abbreviations entirely on the label.
PO and PC mean nothing to most patients, and a patient who cannot read their own label cannot follow it. The label is the one document that goes home with them.
4
Check the duration against the quantity dispensed.
Twice daily for 10 days is 20 tablets. If the quantity says 30, something is wrong — either the sig, the quantity, or your reading of one of them. This check takes two seconds and catches real errors.

Result: A label the patient can follow, and an arithmetic check that the prescription is internally consistent.

Translate element by element, write in the order the patient acts, and always check quantity against duration.

Check your understanding

A prescription is written as “Coumadin 1.0 mg.” Why is this a problem?

Challenge

Apply what you've learned in this lesson.

These need to be automatic, not looked up.

Learn the ten sig abbreviations in the table until you can translate them without thinking.
Find the current ISMP list of error-prone abbreviations. Write out five and what each is mistaken for.
Translate five sigs into patient instructions, then check each quantity against its duration.
Write down what you would do if a prescription arrived using U for units.

Finished this lesson?

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