Error-Prone Abbreviations


The abbreviations found in this table have been reported to the Institute for Safe Medical Practices (ISMP) through the ISMP Medication Error Reporting Program as being frequently misinterpreted and involved in harmful medication errors. These abbreviations should never be used when communicating medical information. This includes internal communications; verbal, handwritten, or electronic prescriptions; handwritten and computer-generated medication labels; drug storage bin labels; medication administration records; and screens associated with pharmacy and prescriber computer order entry systems, automated dispensing cabinets, smart infusion pumps, and other medication-related technologies.

Abbreviations Intended Meaning Misinterpretation Correction
Doses or Measurement Units
cc Cubic centimeters Mistaken as “u” (units) Use “mL”
IU* International unit(s) Mistaken as “IV” (intravenous) or the number 10 Use “unit(s)” (International units can be expressed as units alone)
l Liter Mistaken as the number 1 Use “L”
ml Milliliter Lowercase letter “l” mistaken as the number 1 Use “mL”
MM or M Million Mistaken as “thousand” (M has been used to abbreviate both million and thousand) Use “million”
M or K Thousand Mistaken as “million” (M has been used to abbreviate both million and thousand) Use “thousand”
Ng or ng Nanogram Mistaken as “mg” or “nasogastric” Use “nanogram” or “nanog”
U or u* Unit(s) Mistaken as 0 or 4, causing a 10-fold overdose or greater (eg, 4U seen as 40 or 4u seen as 44); mistaken as “cc” so dose given in volume instead of units (eg, 4u seen as 4cc) Use “unit(s)”
μg Microgram Mistaken as “mg” Use “mcg”
Route of Administration
AD, AS, AU Right ear, left ear, each ear Mistaken as “OD, OS, OU” (right eye, left eye, each eye) Use “right ear”, “left ear”, or “each ear”
IJ Injection Mistaken as “IV” or “intrajugular” Use “injection”
IN Intranasal Mistaken as “IM” (intramuscular) or “IV” Use “NAS” or “intranasal”
IT Intrathecal Mistaken as “intratracheal”, “intratumor”, “intratympanic”, or “inhalation therapy” Use “intrathecal”
OD, OS, OU Right eye, left eye, each eye Mistaken as “AD, AS, AU” (right ear, left ear, each ear) Use “right eye”, “left eye”, or “each eye”
Per os By mouth, orally The “os” can be mistaken as “left eye” (OS) Use “PO”, “by mouth”, or “orally”
SC, SQ, sq, or sub q Subcutaneous(ly) “SC” and “sc” mistaken as “SL” or “sl” (sublingual); SQ mistaken as “5 every”; The “q” in “sub q” has been mistaken as “every” Use “SUBQ” or “subcutaneous(ly)”
Frequency or Instructions
HS Half-strength Mistaken as “bedtime” Use “half-strength”
hs At bedtime, hours of sleep Mistaken as “half-strength” Use “HS” for bedtime
o.d. or OD Once daily Mistaken as “right eye” (OD), leading to oral liquid medications given in the eye Use “daily”
Q.D., QD, q.d., or qd* Every day Mistaken as q.i.d. (four times daily), especially if the period after the “q” or the tail of the “q” is misunderstood as an “i” Use “daily”
Qhs or qhs Nightly at bedtime Mistaken as “qhr” (every hour) Use “nightly” or “HS” for bedtime
Qn or qn Nightly or at bedtime Mistaken as “qh” (every hour) Use “nightly” or “HS” for bedtime
Q.O.D., QOD, q.o.d., or qod* Every other day Mistaken as “qd” (daily) or “qid” (four times daily) if the “o” is poorly written Use “every other day”
q1d Daily Mistaken as “qid” (four times daily) Use “daily”
q6PM, etc. Every evening at 6 PM Mistaken as “every 6 hours” Use “daily at 6 PM” or “6 PM daily”
SSRI Sliding scale regular insulin Mistaken as selective-serotonin reuptake inhibitor Use “sliding scale (insulin)”
SSI Sliding scale insulin Mistaken as Strong Solution of Iodine (Lugol’s) Use “sliding scale (insulin)”
TIW or tiw 3 times a week Mistaken as “3 times a day” or “twice in a week” Use “3 times weekly”
BIW or biw 2 times a week Mistaken as “2 times a day” Use “2 times weekly”
UD As directed (ut dictum) Mistaken as unit dose (eg, order of “dilTIAZem infusion UD” mistakenly given as a unit [bolus] dose) Use “as directed”
D/C Discharge or discontinue Premature discontinuation of medications if D/C (intended to mean “discharge”) mistaken as “discontinued” on a medication list Use “discharge” and “discontinue” or “stop”
OJ Orange juice Mistaken as “OD or OS” (right or left eye); drugs meant to be diluted in orange juice may be given in the eye Use “orange juice”
Period following abbreviations (eg, mg., mL.) mg or mL Unnecessary period mistaken as the number 1, especially if written poorly Use without a terminal period (eg, “mg”, “mL”, etc.)
SS or ss Single strength, sliding scale (insulin), signs and symptoms, or ½ (apothecary) Mistaken for each other or the number “55” Use “single strength”, “sliding scale”, “signs and symptoms”, or “one-half” or “½”

* Included on The Joint Commission’s “Do Not Use” list (Information Management standard IM.02.02.01) and must be included on an organization’s “Do Not Use” list.


Source: Institute for Safe Medication Practices. List of Error-Prone Abbreviations. 2021. Available at: