If you have ever looked at a prescription form — either a handwritten pad or a digital printout — you may have noticed the abbreviation “SIG” followed by a string of shorthand codes. What does SIG mean on a prescription? It comes from the Latin word “signa” or “signetur,” meaning “write” or “let it be labeled.” The SIG is the section of a prescription that tells the pharmacist how to label the medication with dosing instructions for you, the patient. It is, in practical terms, the most important part of your prescription because it dictates how you take the drug. For a full breakdown of prescription anatomy, see our article on what Rx means.
The Anatomy of a Prescription
Every prescription contains several standard components, and understanding them gives context to where SIG fits in. A complete prescription, whether handwritten or electronic, includes:
- Patient information: Name, date of birth, address
- Date: When the prescription was written
- Rx (superscription): The symbol meaning “take” that marks the beginning of the prescription
- Inscription: The medication name, strength, and form (e.g., amoxicillin 500 mg capsules)
- Subscription: Quantity to dispense (e.g., #30)
- SIG (signatura): Directions for use — how much to take, how often, by what route, and for how long
- Prescriber information: Doctor’s name, DEA number (for controlled substances), and signature
The pharmacist reads the SIG and translates any abbreviations into plain English on your medication label. When your bottle says “Take 1 capsule by mouth twice daily with food,” that instruction originated from the SIG field.
How to Read a SIG Code
SIG codes use a combination of Latin abbreviations and standard medical shorthand. Here is an example:
SIG: i cap PO BID c cibo x 10d
Translated: Take 1 capsule by mouth twice daily with food for 10 days.
Breaking it down:
- i = one (Roman numeral)
- cap = capsule
- PO = per os (by mouth)
- BID = bis in die (twice daily)
- c cibo = cum cibo (with food)
- x 10d = for 10 days
While this notation may look cryptic, it communicates precise instructions in a compact format that pharmacists are trained to interpret. The FDA and the Institute for Safe Medication Practices (ISMP) encourage prescribers to write out instructions in full when possible to reduce interpretation errors, but SIG codes remain common in practice.
Common SIG Abbreviations You Should Know
Understanding the most frequent SIG abbreviations helps you verify that your medication label matches what your doctor intended:
Dosage Amounts
- i, ii, iii, iv = 1, 2, 3, 4 (using lowercase Roman numerals)
- ss = one-half (from the Latin “semis”)
- tab = tablet
- cap = capsule
- gtt = drop (from the Latin “gutta”)
- tsp = teaspoon (5 mL)
- tbsp = tablespoon (15 mL)
Frequency
- QD = once daily
- BID = twice daily
- TID = three times daily
- QID = four times daily
- Q4H, Q6H, Q8H = every 4, 6, or 8 hours
- QHS = every night at bedtime
- PRN = as needed
Route of Administration
- PO = by mouth
- SL = sublingual (under the tongue)
- PR = per rectum
- IM = intramuscular injection
- IV = intravenous
- TOP = topical (applied to the skin)
- INH = inhaled
Why SIG Errors Are Dangerous
Misinterpreting the SIG is one of the most common sources of medication errors. A prescriber who writes “QD” (once daily) in poor handwriting can have it misread as “QID” (four times daily) — quadrupling the patient’s dose. The Joint Commission has placed several SIG abbreviations on its official “Do Not Use” list specifically because of confusion risks:
- U (for “units”) can be misread as 0, 4, or cc — prescribers should write “units”
- IU (for “international units”) can be misread as IV or the number 10
- QD and QOD can be confused with each other — prescribers should write “daily” or “every other day”
- Trailing zero (1.0 mg) can be misread as 10 mg — write “1 mg” instead
- Missing leading zero (.5 mg) can be misread as 5 mg — write “0.5 mg” instead
Electronic prescribing (e-prescribing) has significantly reduced these errors by standardizing SIG codes in dropdown menus and eliminating handwriting entirely. According to a study cited by the NIH, e-prescribing reduces prescription error rates by approximately 50%.
What to Do If Your SIG Does Not Match What Your Doctor Told You
If the instructions on your medication label differ from what your doctor verbally told you — for example, the doctor said “twice a day” but the label says “three times a day” — do not take the medication until the discrepancy is resolved. Call your pharmacy first, as the pharmacist can pull up the original prescription and check the SIG. If the pharmacist confirms the label matches the prescription, call your doctor’s office to verify the intended instructions.
These discrepancies happen more often than patients realize. A 2019 study in the journal Pharmacy Practice found that approximately 3% to 5% of outpatient prescriptions contain at least one error, with SIG-related issues being among the most common.
Frequently Asked Questions
Is SIG the same as the medication directions on the bottle?
Yes, the SIG is the source of the directions printed on your medication label. The pharmacist reads the SIG codes from the prescription and translates them into the plain-language instructions you see on the bottle.
Do all prescriptions have a SIG?
Every valid prescription includes a SIG. Without dosing instructions, the pharmacist cannot dispense the medication. If a SIG is missing or unclear, the pharmacist will contact the prescriber for clarification before filling the prescription.
What does “UD” or “ut dict” mean in a SIG?
“Ut dictum” means “as directed.” Some prescribers write “UD” or “ut dict” in the SIG field for medications where they have given the patient detailed verbal instructions, such as a tapering steroid dose. While pharmacies accept this, it provides less clarity on the label and is generally discouraged for safety reasons.
The Bottom Line
SIG is the section of your prescription that contains your dosing directions — how much to take, how often, by what route, and for how long. While your pharmacist translates SIG codes into readable instructions on your medication label, understanding the basics helps you catch errors and verify that your label matches your doctor’s intent. Always read your medication label carefully before starting a new prescription, and never hesitate to ask your pharmacist to explain any instruction you do not fully understand. Clear communication about your SIG can prevent medication errors that are entirely avoidable.