Euthanization, also called euthanasia, is the intentional ending of a life via a drug administered by a professional. When applied to pets, this procedure is often called having the animal "put to sleep."
The euthanasia drug is an overdose of barbiturate. It works by stopping the heart and breathing muscles. The drug can be administered with a needle and syringe or through an intravenous catheter. Some veterinarians administer a sedative tranquilizer prior to euthanization in order to make the pet's final moments more peaceful for both the animal and the owner.
Euthanization is performed for various reasons, including injury and disease. Euthanization is also performed at animal shelters due to overcrowding and a lack of resources.