Microgram (mcg)
A microgram is one thousandth of a milligram — a very small unit used when a milligram would be an awkwardly large number.
One milligram equals one thousand micrograms. So 0.25 mg and 250 mcg are exactly the same quantity written two different ways.
Micrograms are where the most dangerous unit error in this area happens. Reading a microgram figure as if it were milligrams overstates the quantity by a factor of one thousand. Health-safety bodies list the abbreviations for micrograms among those most often misread, which is why careful writing spells the unit out every time.
For example
500 mcg is 0.5 mg. 50 mcg is 0.05 mg. Moving the decimal three places is the whole conversion — which is exactly why an error here is never small.
What it is not
A microgram is not a small milligram in the casual sense — it is a thousandth of one. And 'mcg' is not a variant spelling of 'mg'. If a source uses them interchangeably, stop trusting the source.
Where this comes up: reconstitution, syringe-units