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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 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).
When the Greek lowercase “µ” (Mu) in the symbol µg 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 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.