What is the difference between mg and mcg?

"mg" is the abbreviation of "milligram", which is 1/1000th of a gram.
"mcg" is the abbreviation of "microgram", which is 1/1000th of a milligram.

       1        g = 1 000     mg =     1 000 000 mcg
       0.001    g =     1     mg =          1000 mcg
       0.000001 g =     0.001 mg =             1 mcg

To avoid confusion in text refering to both milligrams and micrograms,
the microgram unit is often written as "µg".

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None - the answer was based on knowledge of the subject.  A useful
reference of SI units can be found at http://www.kok.com/metric.htm