The Core of a Prescription: What is a SIG?
A SIG, derived from the Latin word 'signa' meaning 'to write' or 'to label,' is the part of a prescription that details the patient's instructions for a medication [1.2.6, 1.3.3]. These are the directions you see on your prescription bottle label, telling you how, how much, and when to take your medicine [1.2.1]. While prescribers often use a shorthand of abbreviations, known as sig codes, the final label should be a clear, unambiguous instruction for the patient [1.3.3]. A typical SIG written by a doctor might look like a string of code (e.g., 1-2T Q6-8P), which is then translated by the pharmacy team into plain language like, "TAKE 1 OR 2 TABLETS EVERY 6 TO 8 HOURS AS NEEDED" [1.2.1].
Breaking Down the Components of a SIG
A complete and clear SIG contains several key pieces of information to ensure the medication is taken correctly and safely:
- Verb: An action word like 'Take,' 'Apply,' 'Inhale,' or 'Instill.'
- Quantity: The amount of medication to take at one time (e.g., '1 tablet,' '2 drops').
- Dosage Form: The type of medication, such as a tablet, capsule, or liquid (e.g., 'tablet,' 'capsule,' 'teaspoonful') [1.4.2].
- Route of Administration: How the medication enters the body. Common routes include 'by mouth' (PO), 'in each eye' (OU), or 'under the tongue' (SL) [1.2.4].
- Frequency: How often to take the medication, such as 'once daily' (QD), 'twice a day' (BID), or 'every 6 hours' (Q6H) [1.2.7].
- Duration: How long the treatment should last (e.g., 'for 7 days').
- Indication/Reason: Why the medication is being taken, which helps prevent mix-ups (e.g., 'for pain,' 'for blood pressure').
For example, a prescription SIG of '1 tab po bid x 7d' translates to: 'Take one tablet by mouth twice a day for seven days' [1.4.1].
The Language of the Pharmacy: Common SIG Abbreviations
Many SIG abbreviations are derived from Latin, which was historically the language of medicine [1.2.2]. While modern electronic prescribing (e-prescribing) often translates these automatically, understanding the source can help demystify prescriptions. Below are some of the most frequently encountered abbreviations [1.3.1, 1.3.4, 1.3.8, 1.7.2]:
- po: per os, meaning by mouth
- qd: quaque die, meaning every day
- bid: bis in die, meaning twice a day
- tid: ter in die, meaning three times a day
- qid: quater in die, meaning four times a day
- qhs: quaque hora somni, meaning every night at bedtime
- prn: pro re nata, meaning as needed
- ac: ante cibum, meaning before meals
- pc: post cibum, meaning after meals
- gtts: guttae, meaning drops
- od: oculus dexter, meaning right eye
- os: oculus sinister, meaning left eye
- au: aures utrae, meaning both ears
Comparison Table: Latin vs. English SIG Abbreviations
| SIG Code | Latin Origin | English Meaning |
|---|---|---|
| PO | per os | By mouth [1.2.1] |
| BID | bis in die | Twice a day [1.2.4] |
| TID | ter in die | Three times a day [1.2.4] |
| QID | quater in die | Four times a day [1.2.4] |
| QHS | quaque hora somni | Every night at bedtime [1.2.4] |
| PRN | pro re nata | As needed [1.2.4] |
| AC | ante cibum | Before meals [1.3.8] |
| PC | post cibum | After meals [1.2.4] |
| OD | oculus dexter | Right eye [1.7.2] |
| AU | aures utrae | Both ears [1.7.2] |
The Critical Importance of Accuracy
The accurate interpretation and transcription of a SIG is paramount to patient safety [1.5.7]. A misinterpreted abbreviation can lead to significant medication errors, such as incorrect dosage or frequency, potentially resulting in adverse health outcomes or treatment failure [1.5.2, 1.5.4]. For instance, the abbreviation 'Q.D.' (every day) can be mistaken for 'Q.I.D.' (four times a day), and 'μg' (microgram) can be misread as 'mg' (milligram), a thousand-fold overdose [1.6.4, 1.6.7].
To combat these risks, many healthcare institutions and the Joint Commission have created "Do Not Use" lists for particularly dangerous or ambiguous abbreviations [1.6.6]. The rise of e-prescribing and the use of structured, codified SIG formats are major steps toward improving clarity and reducing errors [1.5.1]. A 2021 study showed that the use of a standardized SIG field more than doubled in a single year, reflecting a significant push for improved prescription accuracy [1.5.1]. Despite these advances, the free-text nature of many SIG fields means that variability and potential for error still exist, making pharmacist verification essential [1.5.3].
Conclusion: A Vital Instruction for Patient Health
The SIG is more than just a line of text on a label; it is the critical instruction that ensures a medication is effective and safe. From the prescriber's shorthand to the pharmacist's translation and the final label on the medicine bottle, every step requires precision and clarity. Understanding the components of a SIG and the meaning behind its abbreviations empowers both patients and healthcare professionals to prevent medication errors and ensure the best possible health outcomes. When in doubt about any part of a prescription instruction, always ask your pharmacist for clarification [1.5.5].
For more information on safe medication practices, a useful resource is the Institute for Safe Medication Practices (ISMP). [Authoritative Link: https://www.ismp.org/]
