An obsolete symbol for the microgram, mmg stands for "millimilligram," that is, 0.001 milligram. This symbol should never be used. The SI symbol for the microgram is µg, and an acceptable alternate (often used in medicine) is mcg
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}