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In the metric system, a microgram (µg; while the recommended symbol in the U.S. when communicating medical information is 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[edit]

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.[1][2] 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 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".[3]

Gamma (symbol: γ) is a non-SI unit of mass equal to one microgram. This always-rare use is deprecated.

See also[edit]