What Does SIG Mean on a Prescription?

If you have ever looked at a prescription — 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. In practical terms it is 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, and 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: The 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: The prescriber’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) both encourage prescribers to write out instructions in full when possible to reduce interpretation errors, but abbreviated 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 prescriber intended:

Dosage Amounts

• i, ii, iii, iv = 1, 2, 3, 4 (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 (or “daily”) = 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

Timing and Other Common Terms

• ac = before meals (ante cibum)
• pc = after meals (post cibum)
• hs = at bedtime (hora somni)
• stat = immediately
• ut dict / UD = as directed
• disp = dispense

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. To reduce this risk, the Joint Commission maintains an official “Do Not Use” list of abbreviations, and the Institute for Safe Medication Practices (ISMP) maintains a longer, voluntary list of error-prone abbreviations, symbols, and dose designations. Frequently flagged examples include:

• U or u (for “units”) can be misread as 0, 4, or “cc” — write “units” instead
• IU (for “international units”) can be misread as IV or the number 10 — write “international units”
• QD, QOD can be confused with each other or with other terms — 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
• MS, MSO4, MgSO4 can be confused between morphine and magnesium sulfate — spell the drug name out

Electronic prescribing (e-prescribing) has reduced many of these errors by standardizing directions in dropdown menus and eliminating handwriting entirely. Research summarized by the National Institutes of Health and other patient-safety bodies has found that computerized prescribing can substantially lower prescribing error rates compared with handwritten orders, though it can introduce new types of errors, such as selecting the wrong item from a menu — which is why a human check by the pharmacist still matters.

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 prescriber 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, since the pharmacist can pull up the original prescription and check the SIG against the label. If the pharmacist confirms the label matches the prescription as written, call your prescriber’s office to verify the intended instructions, because the error may have originated in the SIG itself.

These discrepancies happen more often than patients realize. Studies of outpatient prescribing have found that a meaningful share of prescriptions contain at least one error, and directions-related (SIG) issues are consistently among the most common categories. Catching a mismatch before the first dose is one of the simplest ways patients can protect themselves.

Frequently Asked Questions

Is SIG the same as the medication directions on the bottle?

Essentially, 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 safely dispense the medication. If a SIG is missing or unclear, the pharmacist will contact the prescriber for clarification before filling it.

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 or written instructions, such as a tapering dose. While pharmacies accept it, it provides less clarity on the label and is generally discouraged for safety reasons — ask your pharmacist to confirm exactly how to take it.

Why do prescribers use Latin abbreviations at all?

The abbreviations are a centuries-old shorthand that let prescribers write directions quickly and consistently. Today, safety organizations encourage plain-language directions, and e-prescribing systems increasingly spell instructions out, but the traditional codes remain widely used and worth recognizing.

What does “PRN” mean, and is it safe to take PRN medicine whenever I want?

PRN means “as needed,” but it does not mean unlimited. A PRN instruction almost always comes with limits — a maximum number of doses per day and a minimum interval between them. Read the full label and ask your pharmacist if the “as needed” conditions are unclear.

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 prescriber’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.

Sources

• Institute for Safe Medication Practices (ISMP) — List of Error-Prone Abbreviations, Symbols, and Dose Designations
• The Joint Commission — official “Do Not Use” List of abbreviations
• U.S. Food and Drug Administration (FDA) — guidance on reducing medication errors and abbreviation use
• NIH / National Library of Medicine (MedlinePlus, NCBI Bookshelf) — prescription structure and e-prescribing error-reduction literature