SIG Codes

Role of SIG Codes in Medication Safety

SIG codes (from the Latin signa, meaning "write") are the standardized abbreviations used by prescribers to convey medication administration instructions on a prescription.^[1] For pharmacy technicians, accurate interpretation of SIG codes is critical to entering prescriptions correctly, preventing dispensing errors, and ensuring patient safety. On the Pharmacy Technician Certification Exam (PTCE), SIG-code recognition is a high-yield topic tested under the "Medication Order Entry and Fill Process" domain.^[2]

Mastering SIG codes directly impacts patient care: a misinterpreted frequency (e.g., qd read as qid) can lead to a fourfold overdose.^[3] This guide covers the most frequently tested and clinically relevant SIG codes, organized by category, with exam-focused memory aids and safety alerts.

Standardized Abbreviations by Clinical Function

What Are SIG Codes?

SIG codes are a shorthand system—rooted in Latin and English abbreviations—that tells the patient how, when, and how much of a medication to take.^[1] They appear in the "Sig" field on a prescription label and must be translated into plain-language instructions for the patient.

Common SIG Code Categories

The table below groups the most frequently tested SIG codes by clinical function. Bolded items are high-priority for exam recall.

Systematic Approach to Decoding SIG Strings

Step-by-Step SIG Code Interpretation

When a pharmacy technician encounters a SIG code string, they must systematically break it down to ensure accurate data entry.^[4]

1. Identify the dose – e.g., “1 TAB” or “2 CAP”. Ensure the strength matches the prescribed product.
2. Identify the route – e.g., “PO”, “SL”, “TOP”. Verify it is appropriate for the dosage form.
3. Identify the frequency – e.g., “BID”, “Q8H”, “PRN”. Confirm the interval makes clinical sense.
4. Identify special timing or conditions – e.g., “AC” (before meals), “HS” (at bedtime), “PRN pain”.
5. Translate to plain English – Convert the string into a patient-friendly instruction (e.g., “Take one tablet by mouth twice a day with food”).
6. Check for red flags – Compare against the ISMP Error-Prone Abbreviations list; flag ambiguous or dangerous abbreviations to the pharmacist.^[3]

Common SIG Code Strings (Exam Favorites)

• “1 TAB PO QD” → Take one tablet by mouth every day.
• “1-2 CAP PO Q4-6H PRN pain” → Take one to two capsules by mouth every four to six hours as needed for pain.
• “2 GTT AU QHS” → Instill two drops in both ears at bedtime.
  (Note: AU = both ears; AD = right ear; AS = left ear.)
• “1 APPL TOP BID” → Apply one application topically twice a day.
• “1 INH SL PRN wheezing” → Inhale one spray sublingually as needed for wheezing.

Latin Modifiers You Must Know

• AU – auris utraque (both ears)
• AD – auris dextra (right ear)
• AS – auris sinistra (left ear)
• OU – oculus uterque (both eyes)
• OD – oculus dexter (right eye)
• OS – oculus sinister (left eye)
• AA – ana (of each)
• AD LIB – ad libitum (as desired)

Mitigating Dispensing Errors from SIG Misinterpretation

The ISMP Error-Prone Abbreviations List

The Institute for Safe Medication Practices (ISMP) maintains a list of abbreviations that frequently cause medication errors.^[3] As a pharmacy technician, you must recognize these and flag them to the pharmacist. High-yield exam items include:

• QD / QD – Mistaken for QID; write “daily”.
• QOD – Mistaken for QD or QID; write “every other day”.
• U (for units) – Mistaken for “0” or “4”; write “units”.
• MS / MSO4 – Can mean morphine sulfate or magnesium sulfate; write “morphine sulfate” or “magnesium sulfate”.
• .5 mg (leading decimal without a zero) – Easily missed; write “0.5 mg”.
• SC / SQ – Mistaken for SL or “5 every”; write “subcut” or “subcutaneous”.

Look-Alike / Sound-Alike (LASA) SIG Codes

• QD vs. QID – The most classic error: once daily vs. four times daily.
• BID vs. PRN – Can be confused in hurried handwriting.
• TID vs. QID – Similar appearance; always double-check.

Clinical Consequences of SIG Errors

• Underdosing – Treatment failure, worsening of symptoms.
• Overdosing – Toxicity, adverse drug events, hospitalization.
• Wrong route – e.g., oral solution used as eye drops can cause injury.
• Wrong timing – e.g., taking a statin at bedtime is optimal; taking it in the morning may reduce efficacy.^[5]

Essential Competencies for SIG Code Mastery

What the PTCE Expects You to Know

• Recognize the most common 20–25 SIG codes fluently (see table above).
• Translate a SIG string into plain English—this is a frequent multiple-choice format.
• Identify which abbreviation is error-prone (e.g., “QD” should be avoided).
• Know the Latin origin of common codes—helps with logical recall.
• Understand that SIG codes are part of the prescription label: the technician enters the code, and the pharmacist verifies the translation.^[2]

Memory Aids for Fast Recall

• “BID” = “B” as in “Both” times a day (twice).
• “TID” = “T” as in “Three” times a day.
• “QID” = “Q” looks like a circle with four corners → four times a day.
• “AC” = “A” before “C” (cibum = food in Latin) → before meals.
• “PC” = “P” after “C” → after meals.
• “HS” = “H” for “hora” (hour) + “S” for “sleep” → at bedtime.
• “PRN” = “P” for “pain” or “problem” + “RN” for “as needed”.

Quick Review Checklist

• [ ] I can define at least 20 SIG codes without looking.
• [ ] I can translate a full SIG string into patient instructions.
• [ ] I know which 5 SIG codes are on the ISMP error-prone list.
• [ ] I can identify the correct SIG code for a given clinical scenario.
• [ ] I understand the difference between AU/OU, AD/OD, AS/OS.

References

1. Johnston M, Gricar JA. The Pharmacy Technician: Foundations and Practices. 3rd ed. Pearson; 2019. https://www.pearson.com/en-us/subject-catalog/p/pharmacy-technician-the-foundations-and-practices/P200000001313/9780137531097?srsltid=AfmBOoq-5vBLKitNg-BsVtYpKiAmJzzXqO6yEGDCbiDBQUs2sMC6Zmfi
2. Pharmacy Technician Certification Board (PTCB). PTCB Pharmacy Technician Certification Exam Blueprint. 2024. https://ptcb.org/wp-content/uploads/2025/07/PTCE-Content-Outline.pdf
3. Institute for Safe Medication Practices (ISMP). ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations. 2021. https://www.ismp.org/recommendations/error-prone-abbreviations-list
4. American Society of Health-System Pharmacists (ASHP). ASHP Guidelines on Pharmacy Technician Training. Am J Health-Syst Pharm. 2016;73(12):928-930. https://www.ashp.org/-/media/assets/about-ashp/docs/PELA/ASHP-Pharmacy-Technician-Analysis-Full-Document-FINAL.pdf
5. U.S. Food and Drug Administration (FDA). Medication Errors Related to Drug Names. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/best-practices-developing-proprietary-names-human-prescription-drug-products-guidance-industry