IU vs mg
Two distinct unit systems used in peptide and biologic dosing — IU is a potency-based unit, mg a mass-based unit.
For laboratory and research use only — not for human consumption.
International Units (IU) and milligrams (mg) are two distinct systems for quantifying peptide and biologic doses. Milligrams are a straightforward mass-based unit (1 mg = 10⁻³ g); they are appropriate when the precise molecular identity of a compound is known and assumed to be identical across batches. International Units are a potency-based unit calibrated against a defined biological reference standard, originally introduced for compounds (insulin, growth hormone, gonadotropins) whose biological activity per unit mass varied across early manufacturing processes. For recombinant human growth hormone (somatropin), the WHO international standard defines 1 IU = ~0.33 mg of pure protein; this is the conversion routinely used in GH research and clinical practice (a 3 IU dose ≈ 1 mg). For insulin, 1 IU ≈ 0.0347 mg of human insulin. For peptides of fully defined molecular identity manufactured to high purity (BPC-157, TB-500, GHK-Cu, most research peptides), only mg is used because the unit-of-potency framework adds no value where biological activity per mg is constant. Researchers should not interconvert IU and mg between molecularly distinct compounds — the conversion is compound-specific. Errors of this type are a recognised source of dosing mistakes in research and grey-market peptide use.