Making the decision to put a pet to sleep can be a very difficult one for any animal lover. Livestock farmers deal with this decision on a more regular basis and probably suffer less from emotional attachment. Regardless of the situation the process needs to be painless for the animal. There are a number of methods used to euthanize animals.
If a sick pet, such as a cat or dog, is taken to the vet to be put down it will receive an injection of pentobarbital or sodium thiopental. Alternatively, the veterinarian will administer two injections, first a strong anesthetic to cause unconsciousness and then a lethal injection. Both ways result in respiratory and cardiac arrest, and are very quick and supposedly painless.
Shooting a sick animal may be the preferred way for people who prefer to do the job themselves. The main issue here is for smaller pets, dogs for example, the gunshot needs to be very accurate so as not to wound the animal, causing discomfort. Horses are commonly killed in this way. With horses, the gunshot enters through the forehead and into the spine to eliminate suffering.
Smaller animals are sometimes placed in a sealed container, which then has an anesthetic pumped into it. The initial dose induces unconsciousness and as the process continues the animal essentially overdoses.
Another method of putting small animals to sleep without causing suffering is by breaking the animal's neck. The bones in small animal bodies are weaker and so the process can be carried out very quickly. The breaking of the neck severs the central nervous system so the animal feels no pain.
A common way of killing cattle and other larger animals on a farm is with a captive bolt gun, administered through the forehead. There are several types of captive bolt guns, some of which are also used in slaughterhouses.