So I stumbled on a research paper by Okuda and Takedatsu [1] that uses mμg as a symbol for the mass unit.

I couldn't find anything about this measure unit anywhere except a recommendation not to use it. So how much is 1 mμg? I suppose it means milli-micro-grams, so it should be 1 μg/1000, but it's just a guess.


Okuda, K.; Takedatsu, H. Absorption of Vitamin B12 in a Rectal Suppository. Experimental Biology and Medicine 1966, 123 (2), 504–506. DOI: 10.3181/00379727-123-31527.

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    $\begingroup$ It looks like the authors wanted to use a consistent unit, rather than say the doses were 10 nanograms and 2 milligrams, they went for micrograms for both. $\endgroup$
    – Sam Pering
    Commented Jan 26, 2021 at 11:18
  • $\begingroup$ Is there any chance this is subtle humour? The unit could be read as "mug" when stating it as nanograms, or μmg (micro-milli-grams) removes that possible confusion. $\endgroup$
    – Criggie
    Commented Jan 27, 2021 at 9:11
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    $\begingroup$ This could happen in new publications as well when you use your graphing software incorrectly. You tell the software to graph some value, specify the unit as µg, and when the software gets an input value < 1.0, it might multiply by 1000 and prepend a m to the unit, thus converting 0.67 µg to 670 mµg. $\endgroup$ Commented Jan 27, 2021 at 9:50
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    $\begingroup$ Less than a mug. $\endgroup$ Commented Jan 27, 2021 at 13:52

1 Answer 1


Looks like in 1966 there were still residues of double prefix notation in use.

Here's an entry from Russ Rowlett's compilation of units on ibiblio.org (Rowlett):

millimicro- (mμ-)
an obsolete metric prefix denoting 10-9 or one billionth. This prefix has been replaced by nano- (n-).

Here's what is noted on wikipedia:

Double prefixes
Double prefixes have been used in the past, such as micromillimetres or millimicrons (now nanometres), micromicrofarads (μμF; now picofarads, pF), kilomegatons (now gigatons), hectokilometres (now 100 kilometres) and the derived adjective hectokilometric (typically used for qualifying the fuel consumption measures). These are not compatible with the SI.

Rowlett, Russ. "millimicro-". How Many? A Dictionary of Units of Measurement. University of North Carolina at Chapel Hill. http://www.ibiblio.org/units/dictM.html

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    $\begingroup$ I just wonder, why it was used mcg instead of self-offering mmg for $\mu$g ? $\endgroup$
    – Poutnik
    Commented Jan 26, 2021 at 13:14
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    $\begingroup$ @Poutnik : Once we're in the land of double prefixes, 1 mcg would be 1 milicentigram, equivalent to 10 micrograms. There is a not-universal standard use of "mcg" in parts of the healthcare industry, putatively to minimize misdosing due to handwritten conflation of cursive "m" and mu. Regarding "mmg", the micro- prefix was in widespread use prior to the SI, with published usage back to the 1870s. $\endgroup$ Commented Jan 26, 2021 at 21:02
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    $\begingroup$ @Poutnik From Wikipedia: Microgram: "the recommended symbol in the United States and United Kingdom when communicating medical information is mcg." Perhaps "mc" is closer to "micro", which only has one letter "m". $\endgroup$ Commented Jan 27, 2021 at 12:32
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    $\begingroup$ This should be subtitled "Fun Ways To Abuse Units". I particularly like "hectokilometers". Maybe I'll use this to guilt my daughter when driving her back to college at the end of the week. "Do you have any idea of how far we have to take you? My heavens - it's almost FIVE HECTOKILOMETERS!!!". Fortunately, after 19 years she's used to me... :-) $\endgroup$ Commented Jan 27, 2021 at 12:53
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    $\begingroup$ @AndrewMorton Your point is well taken. The FDA noted as well that mcg is intended as microgram. I learned mcg to stand for millicentigram. Given the discussion here, I'm not so sure anymore. $\endgroup$
    – z1273
    Commented Jan 27, 2021 at 12:54

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