Glossary
Micrograms vs Milligrams
1 mg is 1,000 mcg. Why peptide vials state milligrams, why papers report micrograms, and how to avoid slipping a factor of 1000 during conversion.
A milligram (mg) is a thousandth of a gram, and a microgram — written mcg or µg — one millionth. The factor is fixed, so 1,000 mcg = 1 mg and 1 mcg = 0.001 mg. Vials carry milligram labels because that is the sensible unit for weighing freeze-dried solids, whereas amounts in published studies and at receptor level are generally given in micrograms or nanograms.
Where the mistakes happen
Errors arise in the step from the label unit to the working unit, and misplacing a decimal point here is a thousand-fold mistake, not a tenfold one. Worked case: add 2 mL of bacteriostatic water to a 5 mg vial and the solution holds 2.5 mg/mL, which is the same as 2,500 mcg/mL. So 0.1 mL — ten graduations on a U-100 syringe — carries 250 mcg.
Two habits lower the risk. Do the conversion a single time, when you calculate the reconstitution, and write the result on the vial in mg and in mcg. And on handwritten labels write “mcg” rather than “µg”, because a rushed µ is easily read as m. For the arithmetic, use our converter or the guide to reconstitution maths.
Related terms
molarity · molecular weight · our calculator for reconstitution