It’s supposed to help suppress the overactive immune system response to the saliva the midges produce but I’ve never tried it. It’s also supposed to make the blood taste unpleasant to midges.
It isn’t going to hurt the horse but I would buy a product that’s been developed specifically for horses, try something like Bye Bye Itch
I’ve had best results simply by using a thick barrier cream that keeps the midge saliva away from the skin. Try Killitch