In the metric system, a microgram (µg; in the U.S. recommended when communicating medical information: mcg) is a unit of mass equal to one billionth (1×10−9) of a kilogram, one millionth (1×10−6) of a gram, or one thousandth (1×10−3) of a milligram. The unit symbol is µg according to the International System of Units. In µg the prefix symbol for micro- is the Greek letter µ (Mu).
Abbreviation and symbol confusion
The standard symbol for the microgram (µg) uses the Greek lowercase “µ” (Mu) character. Often the Mu is replaced with the Latin lowercase "u", especially when the Mu symbol is typographically unavailable.
The United States-based Institute for Safe Medication Practices (ISMP) and the U.S. Food and Drug Administration (FDA) recommend that the symbol µg should not be used when communicating medical information due to the risk that the prefix µ (micro-) might be misread as the prefix m (milli-), resulting in a thousandfold overdose. The non-SI symbol mcg is recommended instead.  However, the abbreviation mcg is also the symbol for an obsolete CGS unit of measure known as millicentigram, which is equal to 10 µg.
In the UK, because serious medication errors have been made from the confusion between milligrams and micrograms when micrograms has been abbreviated, the recommendation given in the the Scottish Palliative Care Guidelines is that doses of less than one milligram must be expressed in micrograms and that the word microgram must be written in full, and that it is never acceptable to use "mcg" or "μg".
- "ISMP's List of Error-Prone Abbreviations, Symbols, and Dose Designations" (PDF). ISMP. Retrieved 2011-11-02.
- "ISMP and FDA Campaign to Eliminate Use of Error-Prone Abbreviations". ISMP. Retrieved 2013-12-27.