mg, mcg, mL and units
Four units do all the work here, and two of them look almost identical in writing. Getting them straight is the single most useful thing a beginner can do, because the common error is not a small one — it is a factor of a thousand.
Two measure the substance
A milligram (mg) and a microgram (mcg) both measure how much of the compound there is. One milligram is one thousand micrograms. So 0.5 mg and 500 mcg are the same quantity written two ways — and 0.5 mg is not remotely the same as 0.5 mcg.
This is where the dangerous mistake lives. Read a number written in micrograms as if it were milligrams and you are off by a factor of one thousand. Health authorities have repeatedly warned that mcg is written in ways that get misread, which is why careful sources spell out the unit every single time rather than relying on an abbreviation.
Two measure the liquid
A millilitre (mL) measures volume — how much liquid there is, regardless of what is dissolved in it. A U-100 syringe is marked in units, and those marks are also a volume scale: one hundred units is exactly one millilitre. A unit is one hundredth of a millilitre of liquid, and nothing more.
This is worth reading twice. A syringe unit does not measure the drug. It measures liquid. Two syringes drawn to the same mark can contain completely different amounts of compound if the solutions were made at different strengths. That is why a unit number copied from someone else's post is meaningless without knowing the concentration it came from.