The person who suggested the supplement, “Quit” is right. It does work, I’ve used on several horses and it works every time, though one did start to chew again. I would maybe look into the possibility of pain, maybe ulcers. Been there too and felt horrible that I was mad at the horse for chewing when he was trying to tell me something. Also try salt blocks. All our horses have a white salt block, a red one and a yellow sulfur block. Most people ask what the yellow one is when they visit and I tell them the idea came from an old horseman. Mostly used for cows but is one reason why horses chew wood…for the sulfur content. Hanging a toy in the stall like one of those likit treats helps, most of mine have jolly balls. I’ve used sprays, but the best was cayenne peeper mixed in vaseline and rubbed into where they chew. for outside, you could use a muzzle but I would check for pain first if this is a new vice because putting a muzzle on could stress your horse more. Let us know how it works out.