According to the Humane Society of the United States, the injection of sodium pentobarbital is the most humane way to euthanize an animal. This is because it is the safest method and gives the animal the least amount of stress and physical discomfort. This is an intravenous injection and can be administered through the peritoneal cavity, which is a transparent membrane covering the abdomen in mammals, or into one of the four chambers of the heart. It can only be injected into the heart if the animal is completely unconscious. It is not acceptable to inject under the skin, into muscles, into the lungs, into the kidney or spleen, into the liver or into any other area that is non-vascular.
Carbon monoxide is less acceptable than sodium pentobarbital but can be used conditionally in shelters that do not have access to sodium pentobarbital. It is only humane if it is delivered in a properly maintained, commercially manufactured chamber, and the use of engine-grade gasoline is unacceptable. Carbon monoxide is considered inhumane when euthanizing elderly animals, animals that are less than 4 months old, sick animals or pregnant animals.
Gunshot is most commonly used when euthanizing cattle and is a quick way to euthanize a large animal that cannot be brought to a shelter for lethal injection. Gunshot is most commonly used in cases of an injured animal or emergency situations and is highly discouraged in routine euthanasia practiced in shelters. When euthanizing an injured animal that cannot be brought to a shelter, the gun should be held no closer than 2 inches away from the head. This method can be dangerous to personnel and may cause excessive bleeding from cattle.
The Humane Society of the United States lists sodium pentobarbital as the most humane method of euthanasia. Inhumane euthanasia methods include kill traps, drowning, freezing, caffeine and nicotine, decapitation and nitrous oxide. T-61 is a drug that was removed from the market because it contains neuromuscular blocking agents. Neuromuscular blocking agents cause cardiac arrest and can cause an animal extreme pain.