Mouth ulcers in horses can have many causes. Most of them are serious, and if you should find ulcers or blisters in your horse's mouth, have a veterinarian examine the animal to make a proper diagnosis. In the meantime, there are some simple remedies to help ease the discomfort.
Verify the Cause
One reason you need a veterinarian to examine the horse is because of the possibility of toxicity. If the horse has had blister beetle poisoning, or some other toxic reaction to an ingested poison or plant, any treatment you give in the way of painkillers could actually cause more harm than good. So before you treat the horse with any kind of bute or other drug, make sure a veterinarian has done blood work to verify that your horse is not in organ failure.
Simple mouthwashes are the best treatment. Mixing warm water with salt, then syringing it into the mouth, especially on the areas where the ulcers are the most obvious, will provide some relief and also help the ulcers heal. Flushing the mouth with a liquid antacid ican provide some relief. If the area is a small one, resulting from a wound such as a thorn or cut, you can apply a numbing agent, such as Oragel, to the sore spot. All these are temporary, and just provide relief for some of the symptoms. The real treatment will be the resolution of the injury, virus, or other main cause of the ulcers.
In the case of an illness, such as vasicular stomatitis, the ulcers will heal and go away after 10 to 14 days. If the ulcers are caused from poisoning or toxicity, the resolution may be shorter or longer, depending on the cause and the method of treatment. The main problem during this time will be making sure your horse stays hydrated and gets enough nutrition to keep going. When a horse's mouth hurts, the animal stops eating and drinking. The condition may become severe enough that a hospital stay may be necessary. Keep a close eye on water intake, as dehydration is the first effect you will feel if the horse becomes anorexic.