I can't give you any advice with giving it to a horse, but my mother and I have used it on ourselves.
I used it when I flew over to Germany and was unable to sleep. So I took it before bed to relax and then I slept like a baby.
My mom takes it when she gets really stressed out.
The active ingredient of Quietex is valerian root. It works on some horses and has no effect on others. For most horses you'd give an entire tube 2 hrs before the time you need it. I'd read the instructions to make sure.