Drugs Used to Relieve Pain

Types of drugs used to relieve pain include opioids, nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and alpha₂ agonists. As with any other medication, it is always important to follow your veterinarian's instructions carefully regarding the correct dose to be given.

Opioids

Opioids (such as morphine) are used by licensed professionals primarily for their pain-relieving effects despite some well-known side effects. These drugs are the most effective analgesics available for the treatment of acute pain in many different animals, particularly

Opioids relieve pain by acting on the central nervous system. Their side effects, such as sluggishness (sedation), change in mood (good or bad), and excitement, are related to other central nervous system processes. Different species—and even individual animals within a species—respond to opioids in different ways, so doses often require adjustment. For example, a horse might weigh 10 times as much as a large dog, yet the dose of morphine might be similar for both animals. The effect of an opioid depends on additional factors, including whether the animal is in pain, the overall health of the animal, the administration of other drugs at the same time, and individual sensitivity to opioid effects. For some opioids (such as butorphanol), a higher dose does not always bring more pain relief. This is called the ceiling effect.

Nonsteroidal Anti-inflammatory Drugs and Corticosteroids

For many animals, nonsteroidal anti-inflammatory drugs (NSAIDs) are useful as part of a pain management regimen after surgery or to relieve chronic pain, such as that caused by osteoarthritis. Carprofen, firocoxib, and meloxicam are some examples of drugs in this class. NSAIDs work by reducing inflammation, one cause of pain. They are readily available, relatively long-acting, and generally inexpensive. They can also be given at home after an animal has been released from the hospital. For these reasons, they have long been used for pain relief. Veterinarians may prescribe NSAIDs as one part of the total plan for pain relief, provided the animal does not have kidney, liver, blood clotting, or stomach problems.

NSAIDs intended for use in humans should never be given to your pet unless specifically prescribed by your veterinarian, as even small doses of drugs like aspirin or acetaminophen can cause severe adverse effects in some animals. For example, acetaminophen is toxic to cats and should never be used in this species.

Corticosteroids (such as prednisone and cortisol) also reduce inflammation and provide pain relief. They are used less frequently after surgery because they might weaken the immune system. Other possible side effects of corticosteroids include increased appetite and thirst, and increased need to urinate. Corticosteroids are not used at the same time as NSAIDs.

Alpha₂ Agonists

Alpha₂ agonists are used in large animals such as horses to provide both pain relief and sedative effects during procedures in which the animal remains standing. (Sedatives calm nervousness and reduce irritability and excitement.) Some evidence suggests that the sedative effect lasts longer than the pain relief. Alpha₂ agonists include xylazine, medetomidine, detomidine, and romifidine. Using both alpha₂ agonists and opioids results in pain relief and sedation that are greater than the effects of either drug alone.

Alpha₂ agonists are also used for anesthesia before and during surgery in a variety of animals. However, many veterinarians do not use these drugs to prolong pain relief after surgery or trauma because of the potential for adverse effects. In large animals, alpha₂ agonists can cause excessive sedation (such as sleepiness and reduced mental activity) and loss of muscle coordination. In small animals, pain-relieving doses of alpha₂ agonists also cause deep sedation. If alpha₂ agonists are prescribed after surgery, the dose will be much lower than that used before and during the procedure.

Local Pain Relief

Local anesthetics (such as lidocaine) numb just the area around a wound or surgical site. In large animals, local anesthetics are often used in surgeries—either alone or together with other types of anesthesia—to minimize the potential risks and complications of general anesthesia. In small animals, local anesthetics are usually used for minor procedures such as suturing of cuts. Using local anesthetics before surgery may allow veterinarians to reduce the amount of more powerful general anesthetics needed during longer and more complex surgical procedures. This, in turn, can minimize postoperative pain in animals and lead to a faster recovery.

Last full review/revision July 2011 by Peter W. Hellyer, DVM, MS, DACVA