What are the abbreviations for prescriptions? An essential guide for patient safety

October 8, 2025 •

4 min read

According to the National Institutes of Health (NIH), medication errors can lead to serious patient harm, with misinterpretation of abbreviations being a contributing factor. Understanding what are the abbreviations for prescriptions is a vital step for any patient to ensure they are taking their medication correctly and safely.

Quick Summary

This guide explains common prescription abbreviations, their Latin origins, and what they mean for dosage, timing, and administration routes. It covers frequent sig codes and outlines the critical risks associated with misinterpreting medical shorthand, empowering patients to ask informed questions about their prescriptions.

Key Points

Deciphering Frequency: Common abbreviations like bid (twice daily) and tid (three times daily) indicate how often you should take your medication.
Understanding Dosage: Abbreviations such as mg (milligram) or mL (milliliter) specify the quantity of medication per dose.
Recognizing Administration Routes: The route indicates how to take the medicine; for example, po means by mouth, while IV means intravenous.
Identifying Error-Prone Abbreviations: Certain abbreviations, like U for units and QD for once daily, have been identified as dangerous and are on "Do Not Use" lists due to high risk of patient harm from misinterpretation.
Prioritizing Patient Safety: Active participation, like confirming prescriptions with your pharmacist and using electronic prescribing, helps prevent medication errors resulting from misinterpreted medical shorthand.
Knowing Latin Roots: Many abbreviations derive from Latin, such as bis in die for bid, reflecting the historical language of medicine.

The Historical Roots of Prescription Abbreviations

Prescription abbreviations have a long history rooted in Latin, the traditional language of medicine and science for centuries. The practice began as a way for prescribers to quickly and efficiently write out directions for pharmacists, and for many decades, this shorthand system, known as 'sig codes' (from the Latin signa, meaning 'to label'), was standard practice. While electronic prescribing has largely replaced handwritten prescriptions in many modern healthcare systems, these traditional abbreviations still appear on medication bottles and in medical records. Therefore, recognizing these codes remains an important aspect of being an informed patient.

Deciphering Frequency and Timing Instructions

Some of the most common and vital abbreviations on a prescription relate to the timing and frequency of medication administration. Taking a drug at the wrong frequency can significantly impact its effectiveness and safety. Many of these are derived directly from Latin phrases:

q.d. / qd: quaque die, or once a day.
b.i.d. / bid: bis in die, or twice a day.
t.i.d. / tid: ter in die, or three times a day.
q.i.d. / qid: quater in die, or four times a day.
h.s. / hs: hora somni, or at bedtime.
q.h. / qh: quaque hora, or every hour.
q4h: every 4 hours.
q.o.d. / qod: quaque altera die, or every other day.
p.r.n. / prn: pro re nata, or as needed.
a.c. / ac: ante cibum, or before meals.
p.c. / pc: post cibum, or after meals.

Understanding Routes of Administration

These abbreviations specify how the medication should be taken. Confusion in this area can lead to a patient taking medication incorrectly, potentially causing harm.

p.o. / po: per os, or by mouth.
i.m. / IM: intramuscularly (by injection into a muscle).
i.v. / IV: intravenously (by injection into a vein).
s.c. / subq / sq: subcutaneously (by injection under the skin).
o.d. / OD: oculus dexter, or right eye.
o.s. / OS: oculus sinister, or left eye.
o.u. / OU: oculus uterque, or both eyes.
a.d. / AD: auris dextra, or right ear.
a.s. / AS: auris sinistra, or left ear.
a.u. / AU: auris uterque, or both ears.
gtt / gtts: guttae, or drop(s).
sl / SL: sublingually (under the tongue).

Common Dosage and Quantity Abbreviations

Dosage and quantity notations are critical to ensure the correct amount of medication is taken. These can be particularly prone to dangerous misinterpretation, as highlighted by patient safety organizations.

mg: milligram
mcg: microgram
mL: milliliter (often written as 'cc' for cubic centimeter, a practice discouraged by safety organizations)
t., tbsp.: tablespoon
t., tsp.: teaspoon
cap / caps: capsule
tab: tablet
U: units (a high-risk abbreviation that should be avoided, often misread as a '0' causing a 10-fold overdose)

The Critical Importance of Patient Safety

While designed for efficiency, prescription abbreviations can be dangerous. Misinterpretation, especially of handwritten prescriptions, can lead to serious medication errors. In response, many healthcare systems, including organizations like the Institute for Safe Medication Practices (ISMP), have developed official "Do Not Use" lists to prohibit certain ambiguous abbreviations. The shift towards e-prescribing, where prescriptions are sent digitally from the provider to the pharmacy, is another major effort to reduce errors by standardizing the language used. However, patients should always remain vigilant, particularly with older or unfamiliar records.

Common and Potentially Dangerous Abbreviations


Abbreviation	Intended Meaning	Dangerous Confusion	Safer Alternative
QD or Q.D.	Once Daily	Confused with QID (4 times daily) or QOD (every other day)	Write "Daily"
QOD or Q.O.D.	Every Other Day	Easily misread as QD (daily) or QID (4 times daily)	Write "Every other day"
HS	At Bedtime	Confused with "half-strength"	Write "at bedtime"
U	Units	Mistaken for a '0' or '4', leading to tenfold overdoses	Write "units"
cc	Cubic Centimeter	Mistaken for "U" (units) or a 'u'	Write "mL"
o.d. / OD	Once Daily	Confused with OD (right eye)	Write "Daily"

The Role of Electronic Prescribing

Electronic health records (EHRs) and e-prescribing systems have significantly improved medication safety by eliminating issues with illegible handwriting and automatically flagging potentially dangerous abbreviations. When a prescription is sent electronically, the pharmacist receives the information in a standardized, typed format, reducing the risk of misinterpretation. This technology has helped make prescription writing clearer and more consistent, but patients should still take an active role in understanding their medications.

How to Empower Yourself as a Patient

As a patient, you are the final checkpoint for your medication safety. Always read your prescription labels carefully. When you pick up a prescription, verify the medication name, dosage, and instructions with your pharmacist. If you see an abbreviation you don't recognize or if the instructions seem unclear, ask for clarification. Taking an active role in your healthcare can help prevent potential medication errors and ensure you get the most out of your treatment. For more information and resources on medication safety, consult authoritative sources like the Institute for Safe Medication Practices (ISMP) on their list of error-prone abbreviations.

Conclusion

Understanding what are the abbreviations for prescriptions is a crucial skill for every patient. While medical professionals use this shorthand for efficient communication, it carries a risk of misinterpretation. By familiarizing yourself with common sig codes related to frequency, dosage, and administration routes, you can better monitor your treatment and prevent potentially dangerous errors. Always ask your pharmacist or doctor to clarify any unclear instructions. With the increasing use of electronic prescribing, communication is becoming clearer, but a patient's informed participation remains the cornerstone of medication safety.

Frequently Asked Questions

The symbol 'Rx' is an abbreviation for 'prescription'. Its origin is widely believed to come from the Latin word recipe, meaning 'take', or potentially from an ancient symbol representing healing.

Sig is an abbreviation for the Latin word signetur, which means 'let it be labeled'. It indicates the instructions for how the medication should be taken and is translated by the pharmacist onto your medication bottle's label.

Abbreviations for eyes start with 'o' (oculus), such as 'OD' (right eye), 'OS' (left eye), or 'OU' (both eyes). Ear abbreviations start with 'a' (auris), like 'AD' (right ear), 'AS' (left ear), or 'AU' (both ears).

'PRN' stands for the Latin phrase pro re nata, which means 'as needed'. This indicates that you should take the medication only when you need it, rather than on a fixed schedule.

No, many patient safety organizations, including ISMP, consider QD to be an error-prone abbreviation. It can be easily mistaken for QID (four times a day), leading to a significant dosing error. It is safer to write "Daily" or "Once daily" to avoid confusion.

'a.c.' stands for ante cibum, meaning 'before meals', while 'p.c.' stands for post cibum, meaning 'after meals'. This distinction is important for medications whose absorption is affected by food.

Confirming your prescription ensures you understand the directions correctly and can help catch potential errors before you leave the pharmacy. Even with electronic prescribing, it is a crucial final step for patient safety.