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In the metric system, a microgram (µg; or sometimes: 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 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.