If the horse is body sore from lactic acid buildup in the muscles, then I think it would help the mental state because it would take the soreness out. If your horse is not sore, not sure it would help with calmness alone.....it's for body soreness. It's an electrolyte solution.
No problem. My vet always prescribes it after hock injections to help get rid of the soreness caused by running with sore hocks. I'm sure if her soreness was gone, she would be much more mellow. ;)