In the metric system, a microgram or microgramme (μg; the recommended symbol in the United States when communicating medical information is mcg) is a unit of mass equal to one millionth (×10−6) of a 1gram. 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
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.
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”.
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