In the metric system, a microgram or microgramme is a unit of mass equal to one millionth of a gram. The unit symbol is ?g according to the International System of Units; the recommended symbol in the United States when communicating medical information is mcg. In ?g the prefix symbol for micro- is the Greek letter ? (Mu).
When the Greek lowercase "?" (Mu) in the symbol ?g is typographically unavailable, it is occasionally--although not properly--replaced by the Latin lowercase "u".
The United States-based Institute for Safe Medication Practices (ISMP) and the U.S. Food and Drug Administration (FDA) recommend that the symbol ?g should not be used when communicating medical information due to the risk that the prefix ? (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 ?g.
In the UK, the ?g symbol is the widely recognized method of identifying micrograms.