One of the most critical responsibilities of those in the animal care and sheltering field is to provide the most humane death possible for dogs and cats when euthanasia is necessary. The term "euthanasia" is derived from Greek, meaning "good death." To be humane, every euthanasia technique must result in painless unconsciousness, followed by respiratory, then cardiac arrest, and ultimately death.

Animal sheltering personnel must consider many factors when choosing a method of euthanasia. The most important factor is the humaneness of the method. Other considerations include the number and types of animals handled, the number of employees available, the training available for euthanasia personnel, and legal limitations. Once an acceptable method has been chosen, shelter personnel must carefully maintain euthanasia equipment and keep an accurate inventory of euthanasia drugs to ensure both an adequate supply and the fulfillment of federal and state record-keeping requirements.

It is a binding obligation of shelter administrators to evaluate current euthanasia procedures frequently, ensure that animals are being properly handled, and verify that employees are competent, compassionate, and properly trained. Employees must be able to cope with euthanizing large numbers of animals while maintaining a concern for the well-being of each individual dog or cat. Euthanasia procedures should be clearly documented in writing to ensure consistency.

Sodium Pentobarbital

The Humane Society of the United States (HSUS) recommends the injection of sodium pentobarbital, prepared specifically for use as a euthanasia product, as the preferred agent for the euthanasia of dogs and cats. This method, when properly performed, has been found to be the most humane, safest, least stressful, and most professional choice by The HSUS, the American Veterinary Medical Association, the American Society for the Prevention of Cruelty to Animals, American Humane Association, and the National Animal Control Association.

The HSUS strongly believes that there should be two people involved in the euthanasia process: one to hold and calm the animals and one to inject the drug. Intravenous (IV) injection (into the vein) is considered to be the most rapid and reliable method of performing euthanasia by injection when it can be administered without causing fear or distress in the animal. Sodium pentobarbital may be administered by intraperitoneal (IP) injection (into the peritoneal cavity) to cats, kittens, and puppies if IV injections are difficult or impractical. The use of pre-euthanasia anesthetic drugs should be considered prior to administration of sodium pentobarbital to ensure safe and humane handling of aggressive, fearful or feral animals. Muzzling and other forms of humane constraint may also be used as needed.

Intracardiac (IC) injections (into one of the four chambers of the heart) are acceptable only for animals who have been verified as unconscious. An injection into a conscious animal’s chest is stressful and extremely painful and therefore considered to be cruel. There are two methods of ensuring that an animal is unconscious before administering an IC injection, and both of these methods should be used in every case: 

• Push against the cornea of the eye and check for a blink reflex. If the animal blinks, the animal is not unconscious.
• Pinch the web (tissue between the toes) of the rear paw firmly with the fingers or a pair of hemostats and check for a withdrawal reflex. If the animal pulls back, the animal is not unconscious.

Because intrahepatic (IH) injections (into the liver) have not yet been sufficiently studied, The HSUS is opposed to IH injections. Questions remain regarding the accuracy of injection, organ sensitivity to pain, suitability for multiple species, and smooth induction of the animal into unconsciousness.

In addition, the following euthanasia injection routes are not acceptable: subcutaneous (under the skin), intramuscular (into the muscle), intrapulmonary (into the lung), intrarenal (into the kidney), intrasplenic (into the spleen), intrathecal (into the membranes of the spinal cord), intrathoracic (into the chest cavity), and any other nonvascular injection route.

Sodium pentobarbital is a Schedule II barbiturate, which means it is a federally controlled substance; it can only be purchased using a Drug Enforcement Administration (DEA) registration and order form, and is subject to federal security and record-keeping requirements. Its use is also carefully controlled by state laws and regulations.

Pentobarbital Combinations

Any combination of sodium pentobarbital with a neuromuscular blocking agent is absolutely unacceptable for euthanasia. There are, however, other combination products on the market, which combine sodium pentobarbital with another drug to hasten cardiac arrest. The combination line of products is only approved for use in dogs and can only be administered by intravenous or intracardiac injection. With other injection routes, there are concerns that cardiotoxic properties may cause cardiac arrest before the animal is fully unconscious. Again, intracardiac injections may only be performed when an animal is unconscious (as specified in above section).

Because the sodium pentobarbital is combined with another drug, the DEA has classified the combined product as a Schedule III controlled substance, with less potential for abuse. Schedule III drugs can be obtained directly from the manufacturer by anyone with a DEA registration; no order form is required. However, Schedule III drugs are subject to the same security and record-keeping requirements as Schedule II drugs.

Carbon Monoxide (CO)

Carbon monoxide (CO) is not an acceptable method of euthanasia where sodium pentobarbital can be legally obtained by shelters. The many limitations of CO make the method less practical, considerably slower, and more expensive than lethal injection. For those states where shelters cannot legally obtain, use, and administer sodium pentobarbital, The HSUS considers the use of CO a conditionally acceptable method of euthanasia for some animals when delivered by a commercially manufactured and properly equipped chamber. Only cool bottled, commercial-grade gas must be used; engine or chemically-generated gas is not acceptable due to impurities and heat, which make its use painful and inhumane. The HSUS urges all agencies currently using CO to partner with a local veterinarian who can perform euthanasia by injection and to contact your legislators to inform them of the need for direct licensing to enable sheltering agencies to purchase sodium pentobarbital. It is always unacceptable to use CO for the euthanasia of dogs and cats who are:

• Geriatric;
• Under the age of four months;
• Sick or injured; or
• (Obviously) pregnant.

Old, sick, or injured animals may have poor blood pressure or weak hearts, which may delay the effects of CO, causing them to experience distress prior to unconsciousness. Animals under the age of four months may not have the lung capacity to inhale enough CO to be effective. In pregnant animals, it is likely that the mother will die from exposure to CO before the unborn puppies/kittens. Consequently, it is possible that the puppies/kittens will die as a result of the mother’s death (by suffocating to death) rather than from exposure to CO. The HSUS condemns using CO for animals meeting the above criteria.

Because of these limitations, there must always be an acceptable backup method available, namely sodium pentobarbital. In most cases, agencies without access to sodium pentobarbital will need to contract with a local veterinarian to have sodium pentobarbital administered to these animals. Again, agencies without legal accessibility to sodium pentobarbital should explore the possibility of partnering with local veterinarians to perform routine euthanasia as well.

Human Health Risks Posed by CO

Staff must be fully notified of the potential health risks involved with using CO. Carbon monoxide is a hazardous substance: it is highly toxic and has no odor, no color, and no taste. It is one of the leading causes of accidental poisoning in the United States and repeated exposure to CO, even at low levels, can result in long-term effects, including (but not limited to) impaired memory, breathing difficulties, muscle weakness, heart irregularity, and brain damage. Exposure among pregnant women can cause low birth weight in their infants. The use of a CO chamber should be closely monitored and in compliance with Occupational Safety and Health Administration (OSHA) requirements. Again, CO should be considered only for agencies unable to legally obtain sodium pentobarbital.

The HSUS is working to change the laws to allow all animal care and control agencies to legally obtain sodium pentobarbital (see Direct Licensing Laws for Euthanasia in Animal Shelters). Alert your government officials that the use of CO by animal care and control agencies is being phased out across the nation and replaced with the more humane method of sodium pentobarbital injection. Every effort must be made to ensure the safety of shelter workers and to provide a painless and rapid humane euthanasia for animals.

For specific guidelines on the use of carbon monoxide (CO), see Appropriate Use of Carbon Monoxide for Animal Euthanasia.

Pre-euthanasia Anesthetic Drugs

Pre-euthanasia anesthetic drugs are often necessary to safely and humanely handle excited, fractious, or dangerous animals for euthanasia. The administration of these drugs requires minimal handling of the animal and a simple intramuscular injection. Most pre-euthanasia drugs are Schedule III controlled substances and must be handled and secured to ensure compliance with state and federal laws.

There are a variety of drugs that are commonly used for pre-euthanasia anesthesia. The terms tranquilization, sedation, immobilization, and anesthesia are often used interchangeably. However, it is imperative to understand the differences among these words and which drug(s) offer what effect.

Tranquilization: An animal usually remains awake but is calm, relaxed, and may fall asleep. The animal feels pain. A tranquilized animal can be unpredictable and may have a heightened reaction to a sufficient stimulus, posing potential danger to staff. Acepromazine is a common tranquilizer used with animals.

Sedation: An animal falls into a sleep-like state and becomes uncoordinated with relaxed muscles. There is a decreased ability to feel pain (analgesia). Sedated animals are also unpredictable and may be aroused when stimulated. Xylazine is a common sedative used with both large and small animals.

Immobilization: An animal is paralyzed and unable to move. The body may become rigid and stiff. Generally animals are unresponsive to sight and sound, but they can feel deep pain. Ketamine is a common immobilizing agent used in cats.

Anesthesia: An animal is unconscious, has a total loss of feeling pain, and is immobilized, yet the vital functions remain normal. Examples of anesthetic agents are tiletamine-zolezepam (Telazol®) or a mixture of ketamine-xylazine. Both of these drugs/drug combinations offer good anesthesia and allow for an intracardiac injection of sodium pentobarbital when properly administered.

Inhumane Methods

The HSUS considers inhumane, disapproves of, and campaigns against the following methods when used for euthanasia:

• Gunshot—Gunshot is absolutely not acceptable for routine shelter euthanasia. Gunshot is only acceptable in an emergency field situation where an animal cannot be confined and transferred to the shelter, sodium pentobarbital is unavailable, and the personnel are well trained in the use of gunshot. This method is highly dangerous to personnel.
• Carbon Dioxide (CO₂)—CO₂ is not acceptable for use in animal care and control facilities for the euthanasia of dogs and cats. CO₂ produced from dry ice or generated from any other method is unacceptable.
• T-61—T-61 is an unacceptable injectable drug combination containing a local anesthetic, a general anesthetic, and a neuromuscular blocking agent. It has been removed from the market by its manufacturer in the United States, but is still available in Canada. It is intended to be given intravenously at a precise rate of injection (1cc for 5 seconds) that is almost impossible to maintain. Most importantly, if improperly administered, T-61 can cause animals intense pain after administration and a curare-like paralysis of respiration (suffocation) before the animal loses consciousness. Because of these limitations, T-61 is an unacceptable euthanasia agent.
• Other methods that The HSUS considers inhumane—and which are illegal in many if not most jurisdictions—include decompression, the use of kill traps, nitrous oxide, drowning, decapitation, cervical dislocation, thoracic compression, pithing, exsanguination, electrocution, air embolism, nitrogen flushing, strychnine, chloral hydrate, caffeine, nicotine, magnesium sulfate, potassium chloride, succinylcholine chloride (Sucostrin, U-Tha-Sol, Anectine, Quelicin Chloride, Scoline Chloride), and any combination of sodium pentobarbital with a neuromuscular blocking agent.

For More Information

The Humane Society of the United States is committed to recommending only those methods of euthanasia that are painless, rapid, and—to the fullest extent possible—minimize fear and apprehension in the animal. The HSUS is actively working to change state laws to allow local sheltering agencies access to sodium pentobarbital through direct licensing (see Direct Licensing Laws for Euthanasia in Animal Shelters). If you would like information on how to help in this effort, please contact The HSUS Government Affairs Department at 202-452-1100 or Legislation@hsus.org.

Humane Society University offers euthanasia and compassion fatigue training around the country and The HSUS has published an instructional book on euthanasia entitled The Humane Society of the United States Euthanasia Training Manual. The manual covers euthanasia methods for a variety of animal species including wildlife.

To see the Carbon Monoxide vs. Sodium Pentobarbital Cost Analysis Worksheet, please reference Table 14.1 in The Humane Society of the United States Euthanasia Training Manual. To see the May 2004 investigation of carbon monoxide (CO) exposures related to the use of CO euthanasia chambers by the National Institute for Occupational Safety and Health and the Centers for Disease Control and Prevention, visit www.cdc.gov/niosh/hhe/reports/pdfs/2004-0123-2939.pdf.

For a standard operating procedures sample template for euthanasia and controlled substances, contact our Animal Sheltering Issues at asi@hsus.org.

Last Updated 8/23/05