A vet puts a dog to sleep by first injecting the animal with a sedative to calm him down before giving the dog an overdose of an anesthetic, and the whole procedure is a very peaceful and humane way to euthanize. Understand the process of putting a dog to sleep with helpful information from an experienced veterinarian in this free video on pet health.
Euthanasia is often used obviously for terminal diseases such as cancer, very, very severe arthritis, things of that nature, especially in older dogs. And so what is done when the decision is made to euthanize a pet is your veterinarian may or may not use a sedation first that is giving them an injection of something that can calm them down especially if they are very nervous or something like that, it makes them feel better, it makes things go a lot easier and it just makes everybody feel a whole lot better about the situation. Secondly, if that is done, then an IV or intravenous injection of an anesthetic is given. This anesthetic generally is sodium pentobarbital and it's an overdose of an anesthetic and so what it does essentially is disassociate and turn the brain off from any feeling and subsequently what that does is stop the heart. Some of those products will have potassium chloride in them and that will stop the heart as well and so what you're doing is essentially putting them under anesthesia but by overdose and so therefore it's nice and peaceful, these dogs and cats do not feel anything when this is happening. They only feel the needle prick unless they're under sedation and so again, we try to make it a positive thing for everybody since it's such a hard thing to deal with so therefore we try to make it a peaceful type of procedure. But anyway it's an anesthetic that's given by overdose.