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A microgram (mcg) is a unit of mass equal to one-millionth of a gram.
While the official symbol for the microgram in the International System of Units (SI) is μg (using the Greek letter Mu), you'll often see it written as mcg.
This is particularly common in the medical and pharmaceutical fields.
Using 'mcg' is a crucial safety measure to prevent dosage errors. A handwritten 'μg' could easily be mistaken for 'mg' (milligram), which would result in a 1,000-times overdose—a potentially fatal mistake.
The microgram is the go-to unit for measuring very potent substances, where tiny differences matter for both safety and effectiveness.
It's used for specific substances like:
Measuring in micrograms makes it possible to give an effective dose without risking toxicity.
It's hard to picture how tiny a microgram is.
For perspective, a single grain of ordinary table salt weighs about 500–600 micrograms — meaning a single grain contains more than 500 micrograms.
That helps explain why we measure very potent substances in micrograms: their active doses are tiny.