A milligram (mg) is one thousandth of a gram; a microgram (mcg or µg) is one millionth. The relationship is fixed: 1 mg = 1,000 mcg, and 0.001 mg = 1 mcg. Peptide vials are labelled in milligrams because that is the practical weighing unit for lyophilised solids, while published research amounts and receptor-level quantities are usually reported in micrograms or nanograms.
Why the distinction matters in peptide research
The gap between label unit and working unit is where errors live, and a slip of one decimal place is a thousand-fold error rather than a tenfold one. A worked example: a 5 mg vial made up with 2 mL of bacteriostatic water gives 2.5 mg/mL, which is 2,500 mcg/mL. Withdraw 0.1 mL — ten graduations on a U-100 syringe — and that volume contains 250 mcg.
Two habits reduce risk. Convert once, at the point of the reconstitution calculation, and record the result in both units on the vial label. And write micrograms as “mcg” rather than “µg” on handwritten labels, since a hastily written µ reads as m. The arithmetic is worked through in the reconstitution maths guide and our unit converter.