It's much safer when holding the horse to stand on the same side as the veterinarian so the horse can move away from both of you if necessary. Do not stand directly in front of a horse, as this is just as dangerous as standing directly behind one. Never trust a tranquilizer; although the horse may seem very sedate, these horses can still react to pain or other stimulus and injure you severely.
People tend to relax around a sedated horse and let their guard down, assuming the horse will not move. However, I have seen horses so sedated they could barely stand and yet cause severe injury. So if your veterinarian is working on a sedated horse, stay alert.
Never surprise horses without letting them know you are in the area, especially if they're sedated. You can do this by talking to the horse before touching him. In an emergency situation, stay as calm as possible. In these circumstances a horse is fearful and thus unpredictable, so it is important for the handler to remain calm. This is not easy, but your calmness can help keep your horse calm, and make treatment easier for everyone.