Microgram

In the metric system, a microgram or microgramme is a unit of mass equal to one millionth (10⁻⁶) of a gram. Two different abbreviations are commonly used. The International System of Units (SI) uses μg, where the SI prefix "micro-" is represented by the Greek letter μ (mu). The abbreviation mcg is preferred for medical information in the United States (US), but prescription writing guidance in the United Kingdom advises that "microgram" should not be abbreviated.^[1] A third abbreviation, the Greek letter γ (gamma), is no longer recommended.^[2] The US Institute for Safe Medication Practices (ISMP) and the US Food and Drug Administration (FDA) recommend that mcg should be used, rather than μg, when communicating medical information.^[3] This is due to the risk that μ might be misread as m, for "milli-", which is equal to one thousandth (10⁻³). Such a misreading could result in a thousandfold overdose of a drug or medicine. However, mcg is also the symbol for the obsolete unit millicentigram, derived from the centimetre–gram–second system of units and equal to 10 μg.

Usually, a sequence of the Unicode code point U+03BCμGREEK SMALL LETTER MU followed by the Latin letter U+0067gLATIN SMALL LETTER G should be used. However, if μ is not available it may be represented with U+0075uLATIN SMALL LETTER U or the legacy Unicode symbol U+00B5µMICRO SIGN. In Chinese, Japanese and Korean writing a fullwidth version U+338D㎍SQUARE MU G should be used.^[4]

• List of SI prefixes
• Orders of magnitude (mass), listing a few items that have a mass of around 1 μg.