Can too much biotin cause an itchy rash?

I haven't seen that: I use biotin in my patients with brittle nails and I have never seen a rash with it. The only way to know is to stop the biotin, see if the rash resolves, then start it again, although, if you are truly allergic, this could be dangerous and should only be done under the observation of your doctor.

No: But also note that biotin at any dose is useless. A waste of money. But nobody is allergic to it. It is not toxic. Just useless.