Duffy Duck, the chemical is called T 61, it paralyzes the muscles, but not the brain. It is supposed to be used only on a sedated (sleeping) animal in order to avoid conscious suffocation. However, sometimes the sedation doesn't work that well, or the vet doesn't wait until the sedation actually works. You can't really tell how much the animal suffers once the injection is given since the horse can't move any more. But if it's not asleep, oh my, how horrible. There is plenty of advice on the internet about it, many people think it's perfectly alright to use still when used properly, but I wouldn't risk it. Eutha 77 or similar chemicals, on the other hand, are the same ones used to sedate a horse for surgery. Horse falls asleep, is given more of the chemical until dead. Vets often prefer T 61 because it's much cheaper.