IU

This is one of the two or three most dangerous words in the vocabulary, because it looks like a unit of quantity and is not one. An IU is defined per substance by its effect in a standardised assay, so there is no conversion factor between IU and milligrams that holds generally.

Growth hormone is where people meet it most often, and where the confusion does the most damage: HGH is commonly dosed in IU while nearly everything else on this site is dosed in milligrams or micrograms. A protocol written in one and executed in the other is not a small error.

If a source gives IU, the safe move is to keep working in IU and use the specific substance's stated conversion, not a remembered one.