Is there a plural for microgram?
100 microgram or 100 micrograms?
I think the first is correct one but I am not absolutely sure.

Thank you,
Eve

It's not a word I use much, but I can see no reason whatsoever why it shouldn't be micrograms.

Here's some basics on forming plurals, but I suspect you know all this already.

Still, my bet is micrograms.

Thank you!
Meanwhile I am quite sure that you are right.
Translation from German is confusing, because it is 1 Mikrogramm and 100 Mikrogramm. But I think this is because Mikrogramm is both singular and plural.