What Is “mcg” in a Dosage?

The abbreviation “mcg” stands for micrograms in medical dosages and is sometimes mistaken for the abbreviation for milligrams, or “mg.” One microgram equals one one-thousandth of a milligram, states the U.S. Food and Drug Administration.

The FDA joined with the Institute for Safe Medical Practices in 2006 to reduce misinterpreted abbreviations for medicine and veterinary medicine. Written prescription abbreviations are confusing and a source of preventable medical errors, according to the FDA. Using abbreviations allows a more efficient way to write prescriptions; however, abbreviations are misunderstood due to different veterinary and medical teaching and training and poor penmanship.

The two abbreviation systems used in medical and veterinary schools are the Latin apothecary and avoirdupois systems, explains the FDA. The Center for Veterinary Medicine also found a discrepancy in the amount of medicine patients are prescribed as the result of misinterpreting once a day, abbreviated “SID,” twice a day, “BID” and four times daily, “QID.”

Other prescription issues involve misinterpreting trailing zeros and mistaking the “U” used in micrograms for units. A pharmacist mistaking “5.0” for “50” results in a tenfold increase in the prescribed dosage of medication, reports the FDA. A tenfold increase is also given when the “10U” is mistaken for 100 units.