Animal sheltering personnel must consider many factors when choosing a method of euthanasia. The most important factor is the humaneness of the method. To be humane, every euthanasia technique must result in painless unconsciousness, followed by respiratory, and then cardiac arrest, and ultimately death. Other considerations include the number and types of animals handled, the number of employees available, the training and equipment provided to euthanasia personnel, and legal restrictions. Once an acceptable method has been chosen, euthanasia procedures should be clearly documented in writing to ensure consistency, and shelter personnel must carefully 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 humanely handled, and verify that employees are compassionate, properly trained and afforded essential equipment and supplies. Employees must be emotionally able to cope with euthanasia while maintaining a concern for the well-being of each individual animal.

Sodium Pentobarbital
Carbon Monoxide
Inhumane Methods

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 animal euthanasia. This method, properly performed, has been deemed the most humane, least stressful, safest, 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 animal and one to inject the drug. Intravenous (IV) injection (into the vein) is 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 injection is deemed to be impractical or stressful for the animal. The use of pre-euthanasia drugs is not always necessary but should be considered prior to administration of sodium pentobarbital, to ensure safe and humane handling of certain aggressive or frightened animals. Muzzling and other forms of humane restraint may also be used when needed.

Intracardiac (IC) injections (into one of the four chambers of the heart) are ONLY permitted for animals who have been rendered completely unconscious and insensitive to pain via administration of pre-euthanasia drugs. An injection into a conscious animal’s chest or heart is extremely painful and therefore considered to be unacceptable and cruel. There are two methods of ensuring that an animal is unconscious before administering an IC injection, and both of these methods must 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 for sodium pentobarbital 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.

Sodium 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. These combination products are only approved for use in dogs and can only be administered by intravenous or intracardiac injection. 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

Carbon monoxide (CO) gas is, without question, an unacceptable method of euthanasia in states where shelters can legally obtain and administer sodium pentobarbital. In those states where shelters currently cannot legally obtain and administer sodium pentobarbital or procure veterinary assistance to do so, The HSUS considers the use of CO a conditionally acceptable method of euthanasia for some animals, but only when delivered by a commercially manufactured and properly equipped and maintained chamber meeting the criteria outlined below. Only cool bottled, commercial-grade gas must be used; engine or chemically-generated gas is absolutely not acceptable due to impurities and heat, which make its use painful and inhumane.

It is always unacceptable to use CO for euthanasia of animals who are:

• Geriatric;
• Under the age of four months;
• Sick or injured; or
• 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 (by suffocating) as a result of the mother’s death rather than from exposure to CO. Therefore, The HSUS condemns using CO for animals meeting the above criteria.

The many limitations of CO make it a less humane, less practical, considerably slower, and more expensive method than lethal injection. Because of these limitations, sodium pentobarbital injection must always be available as a backup method. In most cases, agencies without legal access to sodium pentobarbital will need to contract with a local veterinarian to administer it to these animals.

In addition, The HSUS strongly urges all agencies currently using CO to transition to euthanasia by injection or partner with a local veterinarian who can provide this service. Furthermore, it is imperative that these agencies immediately contact state legislators to inform them of the need for a direct licensing law to enable sheltering agencies to purchase and administer sodium pentobarbital

If a shelter has exhausted all possibilities for euthanasia by injection, and carbon monoxide is the only legally available method, the following minimum requirements and conditions must be met:

CO Chamber Requirements

The chamber must be a commercially manufactured unit designed specifically for carbon monoxide euthanasia. “Commercially manufactured” means manufactured by a company whose line of business is specifically the fabrication of carbon monoxide gas chambers or the fabrication of metal machinery. It is unacceptable to use a chamber constructed by any other type of company or individual. Chambers constructed with plywood or cinder block and already in existence may be used only if they are in good repair. Any facility that is considering installing a new chamber or constructing a new building should transition to euthanasia by injection. Furthermore, The HSUS believes it is unacceptable for a facility to install a new gas chamber if it already has legal access to sodium pentobarbital.

The chamber and cylinders must be regularly inspected, with any leaks or damages thoroughly documented and immediately repaired. The chamber must be stationed and operated in a well-ventilated area and exhausted to the outside. The interior of the chamber must be well-lit, and equipped with view ports, a regulator (which maintains the gas concentration), and a flow-meter (which measures the gas concentration). View port glass must be shatterproof and all window and door seals must be intact. All fans, lights or other equipment inside the chamber must be explosion-proof and/or protected by a shatterproof cover. If designed to euthanize more than one animal at a time, the chamber must be equipped with independent sections or cages to separate each animal.

The chamber should achieve a minimum of 6% concentration of gas (the concentration should never exceed 10%, a level at which CO may become explosive) within 20 seconds, the animals must be unconscious within 45-60 seconds, and death must occur within two to four minutes. Monitoring equipment (such as a CO detector with alarm) must be used during operation, and the chamber must be cleaned thoroughly between each use.

The CO Euthanasia Process

Care must be taken to ensure animals are handled and placed in the chamber in the most humane manner possible. Careful use of leashes, catchpoles or other restraint equipment is permitted by trained personnel (catchpoles must never be used on cats). As with euthanasia by injection, it may be advisable to administer pre-euthanasia sedation or tranquilization to frightened or aggressive animals prior to placement in the chamber to ensure humane treatment of animals and employee safety. Animals must never be pushed, dragged, or thrown into the chamber or individual cages.

Different species of animals must never be placed in the chamber together and the chamber must never be crowded. Animals should have enough room to sit or lie down comfortably. A euthanasia technician should be present during the entire cycle in the event of an equipment malfunction or other problem.

The sound of the gas entering the chamber may create fear in some animals, especially cats. They may become restless or aggressive, further stressing other animals. Because the gas does not immediately render the animals unconscious, they may appear to go through a period of resistance and dogs may vocalize or howl and become rigid. These issues further support the stance that all animal shelters using a carbon monoxide gas chamber for euthanasia should make the transition to euthanasia by injection as soon as possible.

Animals must be left in the chamber and exposed to the gas for a minimum of thirty minutes to ensure death and the chamber must be fully exhausted before animals are removed. Death must be verified for each animal prior to disposal. There are four signs that a technician should check to verify death: lack of respiration, lack of eye reflexes, lack of heartbeat, and presence of rigor mortis. Only the fourth sign, rigor mortis, is a certain sign of death. In rare instances, an animal may appear to lack respiration, eye reflexes, or a heartbeat, but still be alive.

For detailed information on euthanasia methods for a variety of animal species including wildlife and for specific instructions on verification of death, please refer to The Humane Society of the United States Euthanasia Training Manual. In addition, Humane Society University offers on-site workshops and online options for euthanasia and compassion fatigue training.

Human Health Risks Posed by CO and Staff Safety

Staff must be fully notified of 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 the leading cause of accidental poisoning in the United States and repeated exposure to CO, even at low levels, can result in many 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 kept in compliance with Occupational Safety and Health Association (OSHA) requirements.

Although some people believe that using CO is safer or less stressful because staff does not have to interact with the animals, placing animals in the chamber does require staff to handle and restrain animals, including those who are frightened or aggressive. The humane handling techniques and administration of drugs recommended in these cases would be the same as those for euthanasia by injection, and therefore may not be cited as an excuse to continue using the chamber.

Pre-euthanasia Drugs

As mentioned above, the administration of pre-euthanasia drugs may sometimes be necessary to safely and humanely handle excited, aggressive, or frightened animals for euthanasia. Regardless, staff should not rely on using these drugs and must instead be proficient in humane handling and restraint techniques and skilled in performing all routes of injection specific to each drug. Finally, staff should not be administering pre-euthanasia drugs to animals simply to avoid giving them direct contact and comfort, or in an effort to minimize their own emotional involvement.

There are a variety of drugs that are commonly used pre-euthanasia. Most are Schedule III controlled substances and must be handled and secured to ensure compliance with state and federal laws. The terms tranquilization, immobilization, sedation, 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.

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.

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.

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, which is the only condition under which it is acceptable to administer an intracardiac injection of sodium pentobarbital when necessary and properly performed.

Inhumane Methods

The HSUS considers the following euthanasia methods to be inhumane:

• Intracardiac injection on conscious animals.
• Carbon Monoxide (CO) where sodium pentobarbital is legally available.
• 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 its use. 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 withdrawn from the market and is no longer manufactured or commercially available in the United States, but is still available in Canada and other countries. 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.
• 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, freezing, 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

One of the most critical responsibilities of those in the animal care and control field is to provide the most humane death possible for companion animals when euthanasia is necessary. The HSUS 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 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 Shelter Services Program at 202-452-1100 or via email.

For a cost comparison, please see the Carbon Monoxide and Sodium Pentobarbital Cost Analysis Worksheets. 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 additional information on euthanasia methods and controlled substances, see our charts on state laws regarding training requirements, access to pre-euthanasia and euthanasia drugs, and carbon monoxide euthanasia. For questions that are not answered here, contact our shelter services experts.

The information in this statement replaces any and all previous recommendations regarding HSUS standards for euthanasia methods. Last updated 3/31/09.