In medical abbreviations, is it mEq or mg that is not to be used?

There was a discussion in one of my pharmacy classes about which one we weren’t supposed to use. My teacher isn’t sure about it either. Does anyone know for sure which one? Maybe someone in the medical field.
mEq stands for milliequivalent and mg stands for milligram. But I thought mg could also stand for microgram. I do know the abbreviation for microgram, but they could get confused between each other.