pain.doc

Pain Control In Dogs And Cats

Ron Hines DVM PhD

Dear Reader,
You should never give your pet the medications I write about without expert, hands-on advice from your local veterinarian. Do not give doses higher than recommended. Although mixing medications is sometimes helpful, it should not be done without expert counsel. This article is for your general knowledge only.

Those of us who have and work with pets know that they experience pain similar to human beings. Although we cannot prove this scientifically, we are so closely attuned to our four-legged friends that we know when they are uncomfortable and troubled by pain. You, as the pet’s owner, are more likely to notice signs of pain than your veterinarian because you are more attuned and bonded to your own pet.

Pain can be a temporary problem or a persistent one. Acute or sudden pain can be the result of surgery or sudden damage to any of the major organs, muscles or bones of the body. However, Pain can be a temporary problem or a persistent one. Some causes of chronic pain in older pets are hip dysplasia, arthritis of the spine and joints instability.

All pain-relieving drugs are called analgesics. Controlling pain in your pet actually improves the outcome of many diseases or surgery. Whenever possible, it is better to give pain-controlling medications early rather than waiting until the pain becomes severe.

It is hard to objectively judge the severity of pain in human beings and even more difficult to do so in animals. Our thresholds to pain differ between one person and another and from one animal to another. Some sources claim a five-fold difference between individual pets. Pain perception depends partially on species, breed, age, gender, time of day and your pet’s individual temperament. Young animals tend to have a lower threshold to pain. Older and debilitated pets are more stoic and may not show as much response to pain - but they feel it just the same. Hunting and working breeds of dogs are more resistant to expressing pain than toy or miniature breeds. Signs of pain are subtler in cats than in dogs. When you take your pet in to an animal hospital, your pet’s is usually worrying about the visit and strange environment will often ignore the pain that you noticed at home. This can be very frustrating for owners who tell me “but he limped at home!”

Cats in pain are more stoic than dogs and they hide pain more effectively. Cats will often hunch up when the pain is in their tummies and be reluctant to move. Some cats hiss if a painful site is touched or become unresponsive to affection and petting. Other cats become aggressive and belligerent when in pain. Often their food consumption goes down. Some cats in acute pain meow pathetically. They often carry their ears down. Many pain control medications are bitter. It can be extremely hard to get cats to accept them. For this reason, medications are best when they are prepared in tasteless capsules by a compounding pharmacy.

In both cats and dogs, pain may lead to over-grooming the area that is painful. This can lead to hair loss and self-mutilation of the area. Some pets tremble and move with their stomachs tensed up. Others tremble. Some will show lameness of an affected leg while others become aggressive, pant or grimace. Any sudden behavior change can be a symptom of pain. Excessive salivation, licking of the lips, dilation of the eyes, rapid breathing and increased heart rate may all be attributable to pain. Some dogs in pain also eat less. Some become restless and do not sleep well. Some stop grooming and appear dejected. Pain can cause an increase in body temperature (fever), respiration, heart rate and blood pressure.

When the pain of hip dysplasia or spinal arthritis in dogs is severe, the dog may be unwilling to rear up on its back legs for a treat. I will often pinch the toe of a pet that appears to be in pain to judge the severity of the pain. If the cat or dog reacts to the toe pinch then I assume the severity of its other pain is less than that of the pinch.

Pain alone can actually change the results of blood chemistry analysis. Dogs and cats in pain may have elevated blood sugar. Their blood cortisol (steroid) and white cell levels often increase. Pain can also interfere with the immune system, increase the risk of infections and slowing the healing of wounds and surgical incisions.

You must understand that complete elimination of long-term pain may be impossible or even undesirable in your pet. But it can be minimized with a number of medications. There are five major classes of medicine that can be used to control pain in dogs and cats. Cats do not metabolize many drugs as well as other animals, so your options are fewer. All of the following medications must be used with extreme caution in cats. You and your veterinarian must weigh the advantages of pain medications in cats against the possible damage that might occur.

There are some general rules in using pain control medications in dogs and cats. The first is to try to give the medication early before the pain becomes too intense. The second is that it is usually safer and more effective to give lower doses of two or more pain control medications that have different modes of action rather than a higher dose of a single medication. Doses should always be kept to the bare minimum needed to give relief. Older patients should receive lower doses less frequently than younger more robust pets. It is also wise to check kidney and liver function before and during the use of pain control medications.

Pain control medications used in pets

The Non-steroidal Anti-inflammatory Agents (NSAIDs):

In 1989 scientists discovered the first of the NSAIDs. Pain sensation and inflammation rely on messenger chemicals called prostaglandins. There are two types of prostaglandins, the “good ones” called COX 1 and the “bad” ones called COX 2. Older NSAIDs prevent the formation both good and bad prostaglandins. “Good prostaglandins protect the lining of the stomach and small intestine and also assist clotting and kidney blood flow. “Bad” prostaglandins cause inflammation, swelling and pain. They have become the most frequently used pain medications used in pets. They are widely used in humans as well. There are basically two kinds of NSAIDs. I call the newer ones (COX 2 blockers)and the older ones (Cox 1 and Cox2 blockers). They both work by blocking an enzyme called cyclooxygenase (COX) which is necessary for both prostaglandin formation. The older NSAIDs cause more problems because they block both the pain-producing prostaglandins and the good, protective, prostaglandins. We suspect that NSAIDs also work directly on the brain to block pain sensation.

Now, the enzyme, Cyclooxygenase occurs in 2 forms: The ”good” COX 1 which controls the formation of “good” prostaglandins that protect the lining of the stomach and intestine, the blood clotting process and blood flow to the kidneys. The “bad” one, COX 2, encourages pain and inflammation. This is a simplified explanation but it should be sufficient for readers. Although all the newer NSAIDs work in the same way, some seem more effective in blocking COX 2 in a particular pet. So don’t give up if the first medication you use doesn’t appear to work.

All NSAIDs are well absorbed when given orally. They are eliminated from the body by the liver and kidneys.

When your pet is in acute (sudden) pain, your veterinarian will often use injectable forms of NSAIDs for fast relief.

The Special Problem Of Cats and NSAIDs

Cats are very special animals. They have a unique body biochemistry and liver function that make the NSAIDs more dangerous to them. Your cat’s liver does not have enough of a specific enzyme (bilirubin-glucuronide enzyme). Because of this, NSAIDs tend to linger in the cat’s blood stream. So NSAIDs must be given in very limited doses and they must be given less frequently.

It appears that two of the few NSAIDs that cats tolerate fairly well are meloxicam and ketoprofen. These medications come in flavored syrup as well as tablets. The volume of syrup needed is small cats so they usually accept it. Both compounds are registered for use in cats in Canada and Europe. There, they appear to be relatively safe especially when given for short periods of time. But neither medication is licensed by the FDA for use in cats in the United States.

Because there is a great deal of variation between individual cats as to the effects of these drugs, your veterinarian should monitor your cat closely for any side effects. This is especially true if either drug is given for extended periods of time.

Possible Side Effects in Pets

Because the liver plays an important roll in eliminating NSAIDs from the body, It is best not to use NSAIDs in the face of known liver disease, because animals with liver disease do not remove these drugs from their bodies normally. Also, on occasion, NSAIDs may cause sudden liver failure. According to the FDA’s records on the toxicity of approved products for dogs, Carprofen (Rimadyl) had had the most problems. However, this may be because it is the most widely prescribed.

From time to time, all of the NSAIDs will cause kidney damage in cats and dogs. This is because they can limit blood flow to the kidneys. This is not a problem if your pet has normal kidneys. But if your pet already has some kidney blood flow damage, more might result. This is especially true with the older NSAIDs. COX-2 enzyme increases blood circulation in the kidneys. When COX-2 is inhibited by the older NSAID versions blood flow through the kidneys can drop to dangerous levels.

Another common side effect is stomach and intestinal bleeding. Again, this is more of a problem with the older NSAIDs. When this occurs, your pet will experience vomiting and diarrhea. If this occurs the medication must be stopped or the dose decreased. This side effect occurs more in pets than in humans that receive NSAIDs. It is especially true of the older NSAIDs, which I listed below. An early warning sign of bleeding is a decreased number of circulating red blood cells. Your veterinarian can check for this periodically.

If I must use the older NSAIDs, I often suggest over-the-counter medications that limit stomach acidity (cimetidine, ranitidine, famotidine) Pantoprazole or omeprazole may also have this potential but I have not used them. These medications may minimize NSAID side effects. Some veterinarians give a synthetic prostaglandin, mistoprostol, with NSAIDs to coat and protect the pet’s stomach and intestines. Omeprazole may also be beneficial but I have found no data on its use in pets.
When administering NSAIDs, do not use two different ones at the same time. They should also not be given in combination with corticosteroids (prednisolone, prednisone, dexamethasone, etc).
Also, pets receiving diuretics such as Lasix (furosemide) are also more susceptible to NSAID side effect.

The Newer And Safer NSAIDs:

Most of this group were originally developed for use in humans. They are safer than the “old” NSAIDs. Because the newer ones are COX-2 selective, they are less likely to cause stomach or intestinal bleeding. This is the reason they were very popular in people until the Vioxx (rofecoxib) scare occurred. Luckily, dogs and cats do not share this risk because they do not commonly have heart attacks. Only Meloxicam injection is approved for short term use in cats.

These are dog or human products. None are approved in the United States for use in cats. But because veterinarians want to relieve suffering, we sometimes give them anyway. There are much fewer options in treating pain in cats. When we do use them in cats, it is called an “off label use”.

Before using these drugs, your veterinarian may suggest bloodwork to determine how well the liver and kidneys are functioning in your pet. If it is for long-term use, I would definitely suggest that. It is generally unwise to give this group of medications at the same time corticosteroid-type medications are given. The exception is in end-of-life situations.

Carprofen (Rimadyl, Pfizer):

Introduced in the USA in 1997 by Pfizer Animal Health Co for use in dogs, this NSAID is very similar to meloxicam. Carprofen is often given to dogs before surgery to decrease post-operative pain.
The other approved use is to combat the pain of arthritis. It is available for dogs in two oral forms, caplets and chewable tablets. In Europe, Canada and other countries, carprofen is also registered for short-term therapy in cats.
It will, on rare occasions (2 per 1000 dogs) , cause liver damage in dogs. Particularly Labrador Retrievers.

Ketoprofen (Orudis, Oruvai, Actron, Oruvail, Orudis-KT, Ketofen) :

This medication is sold for human use, over-the-counter at most pharmacies and super-centers. It is only approved in the US for use in people and horses. In Europe and Canada it is approved for use in dogs and cats. There it is available in tablet or injectable form.
All the factors and precautions I mention in the introduction to NSAIDs apply to ketoprofen.
As with the other NSAIDs, ketoprofen is processed in the liver to inactive byproducts that are eliminated by the kidneys. Side effects can includine intestinal upset with vomiting and diarrhea similar to other NSAIDs. Word-of-mouth and published articles recommend this medication to relieve short-term pain (up to 5 days) in dogs and cats. However, this is not an approved use in the United States. Once given by mouth, ketoprofen is rapidly absorbed. After 2-3 hours, blood levels of this medication are only half their original levels.

Side effects, including liver damage and kidney disease, have been reported in pets. Because ketoprofen can adversely affect blood clotting, I do not suggest it be given before or after surgery.

Etodolac (Etogesic, Wyeth Co):

Etodolac is approved for use in dogs in the United States. It is also quite similar to carprofen. Etodolac is given once a day to manage arthritis. It can be given with or without food.
The most commonly reported side effects to etodolac are diarrhea, vomiting, or mopyness and inactivity. When given at the recommended dose, side effects are rare. But if the dose is trebled, vomiting, intestinal bleeding, and weight loss often occur. Etodolac is eliminated by the liver and with the stool. All the cautions I mentioned in the introduction apply to etodolac.

Meloxicam (Metacam, Mobic, Borringer-Ingelheim, Merial Co):

This potent inhibitor of prostaglandin synthesis is used for the treatment of the acute and chronic pain associated with muscle disease and arthritis. It is also used in the management of surgical pain. Meloxicam is a favorite of mine in this group. This is probably because occasionally I myself have take the human brand of meloxicam called Mobic. It is marketed for use in dogs in the United States. It is available in oral tablet, suspension and injectable forms.
Meloxicam is approved for use in dogs. A larger, loading dose is given on the first day. On succeeding days, the dose should be lowered to the lowest possible amount that keeps the dog pain free or nearly so. Intestinal safety seems to be greater for meloxicam than for many other NSAIDs.

The injectable form of Metacam is approved for cats as a one-time, subcutaneous injection for post-surgical pain. If given more than one time or if other NSAIDs are given, kidney and liver toxicity may occur in cats. Both their kidney and liver function must be monitored frequently. If your veterinarian elects to use it in cats, it should not be given more than two or three days per week. Because of the nature of NSAIDs in cats, you must weight the potential risks against the benefits and make your own decision along with your veterinarian.

Deracoxib (Deramaxx, Novartis):

Deracoxib was first approved for controlling post-operative pain. In 2003, it won approval for the prevention of chronic arthritis pain. It is available in beef-flavored chewable tablets. When used to control the pain of surgery, it should be given by injection about two hours before the surgery. It can then be given for up to six days following the surgery. The dose of deracoxib is lowered when the drug is intended for long-term use in the treatment of arthritis.

Deracoxib is related to a class of antibiotic drugs called "sulfonamides" which means they contains a contain sulfur in their structure. However deracoxib is not an antibiotic. It should not be used in pets that have a history of problems taking sulfas. It should not be given after long periods of anesthesia. Do not use deracoxib in dogs that weigh less than 4 lbs. Do not administer it to pregnant dogs, nursing mothers or dogs under age 4 months of age. It is best to give this medication with food. It should never be given with corticosteroid medications. Do not use deracoxib in dogs that are dehydrated, or taking diuretics, or dogs that have preexisting kidney, liver, heart or circulatory problems. Symptoms of overdosage may include diarrhea, vomiting, and bloody stools.

Duracoxib is unique among the newer NSAIDs in the long length of time it controls pain. It persists in the blood stream longer than other NSAIDs. As with the other NSAIDs, the medication will occasionally cause life-threatening stomach punctures so dogs on this medication need to be monitored closely. It should not be used in dogs with an elevated BUN or Creatinine – signs of kidney disease. In such dogs it can exacerbate kidney failure and uremia. A recent study found that the risk of intestinal perforation was quite high when this product was given at doses exceeding the manufacturer's recomendations or when the dogs received corticosteroids while on the medication.

Studies, using deracoxib in cats have been run. A specially made liquid formula was accepted readily by the cats, and no adverse effects were observed. But several more scientific studies are needed before we know the effectiveness and safety of deracoxib in cats. Until then, you must consider the use of deracoxib in cats as an experiment with unknown risks and benefits.

Tepoxalin (Zubrin):

First marketed for dogs in 2003 by the Schering-Plough Animal Health Corporation,
This medication has properties similar to both carprofen and ketoprofen. This medication not only inhibits “bad” prostaglandin formation but also acts through different pathways to block pain.

Firocoxib (Prevacox):

Firocoxib is similar to dericoxib. It was recently approved in the United States and Europe for the control of pain and inflammation associated with arthritis in dogs. It is available in a chewable tablets.

Tolmetin (Tolectin, McNeil Co, Janssen-Ortho):

I have no experience with this medication. Tolmetin was approved for human use in 1997. It is used to treat rheumatoid arthritis. It is usually taken three times a day. I am not aquatinted with its use in pets. Studies in animals have shown that tolmetin to possess anti-inflammatory, analgesic, and anti-fever activity. In rats, tolmetin prevents the development of arthritis and also decreases inflammation. Tolmetin appears to reduce prostaglandin synthesis similar to other NSAIDs.
In humans it may cause headache, dizziness, nervousness, upset stomach, stomach pain or cramps, vomiting, diarrhea or constipation and gas.

Meclofenamic acid (Arquel):

Meclofenamic acid is FDA-approved for use in dogs This NSAID is available as an oral tablet. Meclofenamic acid has a therapeutic index that is lower than that of other NSAID, possibly due to the way it circulates in the liver. This means that the necessary dose for pain relief is quite close to the dose that can cause side effects. It is sold for the treatment of pain and inflammation - especially that associated with arthritis. It may take three or four days for pain relief to be seen in your dog. Side effects that can occur include vomiting, diarrhea, lack of appetite, bloody stool, black tarry stool, or ulcers in the stomach or small intestines. Less common side effects are depression, fever, behavior changes, fast breathing, edema inability to control urine, or irreversible anemia.

Do not use Meclofenamic acid in dogs that are hypersensitive (allergic) to this drug or any other NSAIDs. Do not use the medication at full dosage for more than 5-6 days. After that period, the dose should be decrease to the minimum dose and most infrequent administration that controls your pet’s pain. That is, give it only a few days per week if possible.
All the earlier warning I have given concerning NSAIDs apply to meclofenamic acid. Do not use this drug in dogs that have kidney or liver problems or heart disease. Tests must be run by your veterinarian to rule these conditions out. Give with a full meal. Do not give meclofenamic acid for a week prior to surgery or the week after surgery. Do not give to dehydrated pets or those taking diuretics for heart or lung problems. Do not use this product in dogs with clotting deficiencies such as Von Willebrand’s disease. Do not give the medication to pregnant or nursing mothers. Do not give to pets under eight months of age. This product is not approved for use in cats. Do not give with other NSAIDs or sulfa antibiotics, glipizide, or valproic acid or oral anticoagulants. In epileptic dogs, Meclofenamic acid may increase blood concentrations of phenytoin. If you exceed the recommended dosage you may see evidence of stomach ulcers and kidney damage.

Vedaprofen (Quadrisol-1 and Quadrisol-5, Intervet Co):

I have no personal experience with vedaprofen. However, reports out of the Netherlands, Portugal and Asia suggest to me that this medication may have significant potential uses in dogs and possibly cats. But because all NSAIDs have similar effects on the body, I do not expect it to be free of the side effects present with all NSAIs.

Intervet, on its European site, states that Quadrisol “has been tested in numerous field trials and has been proven to be safe, effective and well tolerated by dogs” and is said to be safe for use in nursing mothers. However, the possible side-effects are the same as the other newer NSAIDs. It is marketed for dogs for the relief of pain and control of inflammation associated with short-term injury or long-term for arthritic problems. It comes in gel form in a dosing syringe. It should be given with food. It is sold in two formulations: Quadrisol-1 is for use in dogs weighing under 5 pounds and Quadrisol-5 for dogs weighing over 5 pounds. In some countries Quadrisol-1 is also marketed for the management of fever and post-operative pain in cats. In studies in the Netherlands it was found to be a bit more effective than meloxicam.

Celecoxib (Celebrex, Pfizer Inc):
This drug is not approved for use in pets. In one study in beagles, there was dangerous variation in the length of time the drug stayed in the body as well as great variation in blood levels between dogs. I do not recommend its use until we understand it better.

Valdecoxib (Bextra):

This medication was introduced to the US market in 2001 as an arthritis remedy for humans. Valdecoxib is a potent and specific inhibitor of cyclooxygenase-2 (COX-2). However, side effects in humans led to it’s being banned in several countries. There is no data on its use in animals.

The Older, Less Safe NSAIDs:

These medications are much less specific than the newer NSAIDs. These medications reduce both the “good” and the bad prostaglandins. Bleeding is the most common side effect of these drugs. Despite this common side effects, the older NSAIDs are still used today in veterinary medicine because they are so much cheaper than the newer ones. Do not give these older medications after major surgery because they can lengthen the time that wounds bleed. Most dogs receiving these medications eventually develop gastro-intestinal problems and must stop taking the medications.

Aspirin (acetylsalicylic acid):

In 1899, the German company, Bayer, began marketing the new drug "Aspirin". Over the succeeding hundred years, aspirin became the most widely used anti-inflammatory drug in humans and dogs. It can be purchased in various forms including plain, buffered, and enteric-coated formulations as well as topical creams and rectal suppositories. It begins reducing pain in 1-2 hours after it is swallowed. Aspirin like drugs are called salicylates.
None of them, including aspirin, were ever approved by the FDA for use in pets. Aspirin is metabolized and eliminated by the kidneys after being processed in the liver. Before the newer NSAIDs were available, it was commonly used in dogs. It is more dangerous in cats, because they lack a liver enzyme, glucuronyl transferase. Because of this, cats have difficulty processing and eliminating aspirin. Aspirin lingers very long in the blood stream of cats (40hrs). Because of this, I would never give aspirin to cats - but some veterinarians do give the buffered form to cats. No human pill-form of aspirin should be given whole, to small pets. In dogs aspirin is eliminated within 7.5 hrs. Veterinarians used aspirin for the relief of pain associated with muscle or bone inflammation or arthritis. Aspirin should never be used in pets suffering from kidney disease or high blood pressure. I rarely give it because of the high rate of side effects from prolonged use. In cats, it has been used every two days to prevent and dissolve blood clots. Misoprostol helps in reducing stomach and intestinal ulceration associated with aspirin. Aspirin overdose in dogs or cats will result in salicylate poisoning. This is characterized by hemorrhage, severe blood acid-base abnormalities, coma, seizures, and death.

Ibuprofen (Advil, Motrin, Nuprin, Medipren):

Ibuprofen is an arylpropionic acid derivative that has been used in dogs as an anti-inflammatory agent. It is not approved for use in dogs or cats. Dogs are much more likely to develop gastro-intestinal side effects from ibuprofen administration than are humans. For this reason, I never give this medication to pets and don’t advise you doing so. At therapeutic doses, adverse effects observed in dogs include vomiting, diarrhea, gastro-intestinal bleeding, and kidney infection.

Phenylbutazone (Butazolodine, “bute”):

Phenylbutazone is another older NSAID agents that has been used in veterinary medicine for over thirty years to treat arthritis. It is primarily used in horses but was also administered to dogs. Veterinarians rarely administer it today. It is approved by the FDA for pain control in dogs but not cats. Its side effects in dogs can include ulceration and bleeding of the stomach and intestines as well as anemia. In dogs, phenylbutazone has been associated with bleeding disorders, liver damage, kidney damage, and rare cases of irreversible bone marrow suppression leading to death.

Piroxicam (Feldene, Pfizer):

Piroxicam is an older NSAID that is a member of the oxicam group of drugs. It is approved for use in humans only. Although it works well in humans I have found it to cause stomach distress in most dogs that I have tried this medication on. Piroxicam undergoes extensive recycling and processing in the liver of dogs This results in a prolonged presence in the plasma of dogs. Gastric and intestinal ulceration and bleeding and kidney damage have been observed in dogs receiving piroxicam.

Naproxen (Naprosyn. Aleve, Roche Co):

This NSAID is not approved for use in pets and I have never given it. One dose is said to lasts 45-92 hours in dogs. Dogs are extremely sensitive to its toxic effects. So, wjem it is used, it should only be given every second or third day. I am not familiar enough with this drug to suggest a safe dose. Because of the bad side effects that often occur, I do not recommend using naproxen in pets.

Flunixin meglumine (Banamine, Shering-Plough):

Flunixin meglumine is a potent injectable NSAID, which is particularly good for intestinal pain. In the United States, it is approved only for use in horses. Veterinarians have use it frequently in treating the pain associated with parvovirus intestinal disease in dogs and for treating post-surgical stomach pain. However, leading textbooks suggest it not be used at all in dogs. When given, it should not be given for more than two days. Recent research has shown that it can retard healing in dogs – especially when the intestines have been cut. The pain reducing effect of an injection of flunixin meglumine only lasts for a few hours. Long term administration of flunixin meglumine to dogs results in severe gastro-intestinal ulceration and kidney damage. It should not be given more than once or twice.

Ibuprofen (Advil, Motrin, Nuprin, Medipren):

This is a great anti-inflammatory drug in humans but it consistently causes ulcers in dogs after 2-6 weeks of use. In dogs, it will eventually cause ulcers of the stomach as well as vomiting. At a dose low enough to not have these side effects, the drug probably does not relieve pain. I do not recommend the use of ibuprofen in pets.

Indomethacin (Indocin, Indocin-SR):

When it is given to pets at doses high enough to relieve pain, Indomethacin is highly toxic to the gastro-intestinal tract of dogs. It often results in severe ulceration, bleeding, and dark, bloody stools. Do not use it in pets.

Pain Relievers Unrelated To NSAIDs :

Acetaminophen (Tylenol):

Acetaminophen is a para-aminophenol derivative with anti-fever and pain control activity activity, but very little anti-inflammatory effect. Acetaminophen does not produce stomach ulcers or retard blood clotting. Acetaminophen is more effective in inhibiting COX enzymes in the brain rather than in the body. I have never found it to be effective in reducing pain in dogs. In dogs, dose-dependent bad side effects include depression, vomiting, and destruction of blood hemoglobin. It should never be use in cats due to their lack of liver glucuronosyl transferase and the potential for hemolytic anemia and liver destruction. I have read that one extra-strength Tylenol will kill a cat.

Methocarbamol (Robaxin):

Methocarbamol is a muscle relaxant that exerts its effect by acting on the central nervous system (the nerves that control the muscles) rather than on the muscles themselves. It is a rather save drug. Although it does not directly lessen pain, it may relieve muscle tension associated with arthritis in pets. It has weak sedative properties and may make the urine appear darker.

Dissociative Anethetic Agents:

Ketamine (Ketaset, Vetalar):

This medication is commonly used as a general anesthetic in cats. However, studies have found that it reduces pain when it is applied to the skin as a gel or paste. A specialty compounding pharmacy can provide it. There have been some studies that found that ketamine, given by mouth, also alleviates pain. Of course, these studies work best in humans who can tell you if they are feeling less pain. It is also an NMDA receptor antagonist. NMDA is involved in the sensation of pain.

Alpha-2 agonists or Blockers:

Xylazine (Rompun):

Xylazine is an injectable sedative and anesthetic that is approved for use in dogs. It belongs to a class called alpha-2 agonists. When given by intramuscular injection at less than anesthetic doses, it is a very effective pain reliever. Anesthetic doses commonly cause dogs to vomit, but at low dosages, vomiting
and decreased breathing are rare.

I like to incorporate xylazine into my anesthetic protocols for dogs, because pain relief persists long after the surgery is completed. I have not personally used this xylazine in cats for pain relief but several sources suggest dosages that can be used in cats. I, personally, would not give xylazine to cats.

Medetomidine hydrochloride (Domitor, Novartis Co):

Medetomidine hydrochloride is approved as an anesthetic and pain reliever for use in dogs recovering from surgery. It also belongs to a class of drugs called alpha-2 adrenoreceptor agonists. At anesthetic doses this medication slows heart rate.
It is metabolized but the liver and excreted through the kidneys so it should not be given to dogs that have liver or kidney disease. It should also not be given to dogs at times that they are agitated and fearful at the animal hospital or dogs that might go into shock due to coexisting diseases.

NMDA Receptor Antagonists or Blockers:

NMDA receptor antagonists block pain by binding to the N-methyl-D-aspartate (NMDA) receptor. NMDA blocking medications include ketamine, dextromethorphan, memantine, and amantadine. The opioids methadone, dextropropoxyphene, and ketobemidone are also antagonists at the NMDA receptor.

Amantadine (Symmetrel):

Amantadine is the most commonly used oral drug in this class. It was originally developed as an antiviral compound, and has also been used to treat drug reactions that affect coordination (extrapyramidal reactions) and Parkinson's disease in humans. It appears to effectively block pain in dogs. There is positive information on its use in cats but I have never used it in cats. The dose of amantadine must be reduced in pets with poorly functioning kidneys. It has been given on a daily basis, but in most cases, it is given for one to two weeks and then discontinued until p...