Medications and Cats - Side effects, Reactions,Intolerance


United States Pharmacopeia's Veterinary Practitioner's Reporting Program

When an adverse reaction to a medication occurs it is best to report it to the drug manufacturer and the FDA (medications) or USDA (vaccines). To make reporting easier, the United States Pharmacopeia has established a central reporting service. If a veterinary practitioner reports a drug reaction, the USP-VPRP reports to the drug manufacturer, the FDA or USDA and the American Veterinary Medical Association with one phone call to 800-487-7776. The web site for this organization is

Long term use of cortisones

Question: Dear Dr. Mike,

Two years ago, I wrote to you regarding our cat, Equus. We adopted he and his brother from a no-kill shelter at 11 weeks. They were healthy (except for a tendancy to be overweight) but did have some problems with their teeth (our vet called it 'sheltermouth' -- teeth not developing properly in stray kittens due to lack of nutrients).

At 4 years of age, our vet decided to pull teeth. Equus had all of his back teeth pulled. Over the next 4 months, he lost 7lbs. The vet was flummoxed. Then, he began to vomit profusely. Two days later, I woke up in the middle of the night to find him curled on my legs -- burning hot. He was weak and lethargic. I took his temperature and it was 106!! I panicked and called the vet. I ended up on the phone with the vet for 30 minutes while I held Equus, wrapped in freezing cold, wet towels.

For two months, Equus faded away. We tried everything, to no avail. Finally, our vet thought he found Hemobartenella in his blood smears and we started treating. He sent the blood to the U. of Colorado for testing. It came back negative. At our wits end and fairly sure we were going to lose our beloved boy, we agreed to let them do exploratory surgery. They took stomach lining sample and finally were able to diagnose Equus with Inflammatory Bowel Disease (although it was a difficult case to diagnose, even after staring at Equus' insides.)

Equus is doing well now -- though, he still has bouts of vomiting from time to time. We are fairly a well-oiled machine in dealing with it. A week of prednisone and he is back to normal. And here is my only question: long-term prednisone? Can we expect Equus to live a long life (barring any other health-problems)? Or, does long-term low-dose predinisone usually affect the liver too extensively?

Anyway. I thought I would let you know how helpful you were during this panicked and seeminly hopeless time. And, if it helps, here are Equus' symptoms and our treatment machine, just in case anyone out there is dealing with something similar. There is hope!

Weight Loss Depression Vomitting -- regurgitation, in our case. Constipation, sometimes with blood (irritation related) High Fever (only happened once)

*Equus eats probably 7 times a day, we feed him whenever he shows interest. He eats pretty well. We have him on Limited Diets Canned Rabbit. He will instantly regurgitate anything else.

*We are very careful not to leave any other food out that he can get into. Once in awhile, he gets into things anyway (this cat is a genius and a junkfood addict). This last flair-up was brought on by a piece of barbecue chicken wing he got out of the very bottom of the trash.

*We bought a water-fountain style cat water dish with a filter in it. He has had less frequent flair-ups, though he still does have them about every 3 months.

*We keep their box immaculately clean. This also seems to help for some reason.

*During a flair-up, he gets half a 5mg prednisone two times a day for 5 days, and then one time a day for 5 days. He also gets a quarter tab of tagamet in the morning.

Anyway, again -- thank you, so much. I cannot tell you how distraught we were when I last wrote. We have a happy home again.

Always Fondly, Stephanie

Answer: Stephanie-

I am glad that Equus' problems are under control. You have worked hard to make that happen and Equus is fortunate that you choose to adopt him.

Cats are pretty resistant to the side effects normally associated with prednisone usage in humans and dogs but can develop clinical signs of hyperadrenocorticism, such as hairloss, increased drinking and urinating, pendulous appearing abdomen, with continuous use of corticosteroids over a long time span. This usually happens when using long acting injectable corticosteroids at frequent intervals, though. Dogs are much more prone to developing "iatrogenic", or medically induced hyperadrenocorticism, due to cortisone administration.

Liver problems associated with cortisones are also not reported as often in cats as often as they are in dogs and liver problems severe enough to be life threatening are rare even in dogs. There are elevations in serum alkaline phosphatase (SAP) associated with the use of cortisones. Since this enzyme is also elevated in liver disease, there is the mistaken impression that the cortisones cause liver damage frequently. There is a similar impression that kidney damage may be occurring due to increased drinking and urinating. Both impressions are false. Actual liver damage from corticosteroids does happen but it does not happen frequently.

To sum this up, cats are pretty resistant to the side effects of cortisones. If you can successfully use oral prednisone to control the inflammatory bowel disease and you can use it in a "pulse" treatment manner, as you describe, it is very unlikely that Equus will suffer long term problems from the corticosteroid. Even long term use of prednisone on an every other day basis (NOT daily) in cats is unlikely to cause clinical signs of hyperadrenocorticism or liver damage. There is an increased tendency to develop diabetes with long term alternate day use of prednisone in cats, though. So try to stick with the occasional therapy (pulse therapy) approach that you are currently employing for as long as possible.

You have a good relationship with your vets, so I am sure that they will keep you informed of any changes that worry them, such as hairloss or a more pendulous look to Equus' abdomen, and you should do the same.

Mike Richards, DVM 4/28/2000

When Good Medications Do Bad Things

There are a number of medications which are very helpful for most pets but which make a few pets ill or even cause death in extreme instances. Using these medications is often necessary in spite of the possible side effects, especially when there are no other medications for a condition. In other cases, the beneficial effects may justify some risk taking even with serious potential consequences. Being aware of the potential for danger can help to prevent problems or to allow a pet owner or veterinarian to catch them early enough to reverse problems induced by the medications.

The ultimate example of a medication that has harmful side effects but is essential for life, at least in diabetic pets, is insulin. Even a small overdosage can have serious consequences and strict monitoring of the medication is essential. Despite that, there are few calls for it to be removed from the market. Why? Because the benefit clearly outweighs the risk. Where there is no other choice a huge risk is worth taking. For most other medications the risk to benefit picture isn't quite so clear.

Antibiotics are often harmful to patients. Sulfa/trimethoprim (Bactrim Rx, Tribrissen Rx, Ditrim Rx, Sulfatrim Rx, SMZ-TMP, other generic names) is an antibiotic that can cause joint inflammation in Dobermans and is implicated in immune mediated thrombocytopenia (ITP) in many dog breeds. Thrombocytes are the platelets in the blood, responsible for blood clotting. Thrombocytopenia is a decrease in the number of platelets. Obviously, if they get low enough there is a great risk to the pet. This antibiotic is still widely prescribed, though. It has a broad spectrum of action, it is inexpensive and most of the time it doesn't cause problems. The ITP is almost always reversible if the medication is withdrawn. Remembering that this antibiotic can cause this problem may help to save your pet's life, though. Penicillins can cause severe allergic reactions, even causing sudden death in a few patients. Many antibiotics cause diarrhea. Chloramphenicol has been associated with aplastic anemia in several species. Enrofloxacin (Baytril Rx) and tetracycline antibiotics should not be given to growing pets unless absolutely necessary due to the potential for problems with absorption of the medications into bone and/or teeth, causing defects. Amikacin and gentamicin are aminoglycoside antibiotics. This group of antibiotics can cause deafness and kidney failure. Use of antibiotics should be restricted to conditions which are likely to respond to appropriate antibiotic therapy since these are not harmless medications. When they are necessary it is obvious that some risk of use is justified.

Heartworm preventatives often come under scrutiny by pet owners when a pet suddenly dies or develops an illness that may be associated with drug reactions and the only medication the dog is taking is the heartworm prevention medication. Filaribits Plus (Rx) can cause an idiosyncratic (we don't know why it happens) liver reaction in a small number of dogs. Dobermans seem to react more commonly to this medication than other breeds, too. There are alternative heartworm preventatives, so it is possible to use another medication if reactions occur. The newer monthly heartworm preventatives are often suspected of being the cause of the problem when immune mediated hemolytic anemia occurs in dogs but there is no proven connection that I am aware of. About half the cases of immune mediated hemolytic anemia (IMHA) occur for no apparent reason but the most commonly identified reason is probably reaction to an administered medication. Therefore, people are suspicious of the monthly heartworm preventatives when IMHA develops in their pet. Heartworm is a devastating disease and if you live in an area in which it occurs it is essential to use the best medications to prevent it. For most people this is one of the monthly pills (Heartgard Rx, Interceptor Rx and ProHeart Rx). Don't risk heartworm disease in your pet because you run across suggestions of danger on newslists or from other sources.

What about situations in which there is very little risk of death from a condition but there is a great deal of suffering associated with it, or when a medication is used to lessen the stress and anxiety of surgery or examination procedures?

The newest example of a medication with peculiar side effects in a small number of pets is carprofen (Rimadyl Rx). This medication is very effective at controlling pain and allowing dogs with arthritis to move comfortably again. It is safer than most medications in its class for use in dogs since it is much less likely to cause ulcers than other non-steroidal anti-inflammatory (NSAID) medications approved for use in dogs. However, it can cause toxic liver reactions in a few dogs. At the present time the majority of dogs affected in this manner have been Labrador retrievers but other breeds have been affected. There are a small number of confirmed cases of problems and a large number of dogs on this medication so the problem seems to be rare but definitely real. If your dog is on Rimadyl and you see any signs of inappetance, lethargy, vomiting or ill health in general, please contact your vet. Checking to see if there is evidence of liver damage would be a very good idea if any reaction to this medication occurs and it may not be a bad idea to just routinely run tests for liver enzymes a few days to a few weeks after starting Rimadyl. In any case, don't simply assume that whatever is wrong will get better in a few days and keep giving the medication. Stop, call your vet and inform him or her of the problem. Rimadyl is a very good medication and I have used it in one of my own dogs. Just be aware of the potential for problems.

Acepromazine is frequently used by veterinarians to lessen the stress and anxiety associated with anesthesia and for other beneficial effects in anesthetized patients. There are a few reports of serious side effects in boxers. It can also lower the seizure threshold and should not be used in pets known to have seizure disorders. Make sure that the vet or the veterinary assistant notes on the record or admission form that your pet suffers from seizures before a surgical procedure, just in case your vet uses this medication as a standard part of the anesthesia protocol.

There are other peculiar drug reactions and other medications with side effects. We are going to cover the side effects of corticosteroids later in this newsletter and hope to continue to keep you informed of potential problems with medications in future issues. Good medications must still be used carefully.

Mike Richards, DVM

Depomedrol - long term problems

Question: Dr. Mike,

When using Depomedrol on a regular basis I understand that the immune system can be compromised. What type of problems can occur? What symptoms should I look for? How long can I expect this treatment to be beneficial to the quality of life for my 15 year old Himalayan cat?

Thanks for any help, Carol

Answer: Carol-

Even though immune system compromise probably occurs in almost all cats that are treated with DepoMedrol (Rx), we don't see complications from the immune suppression very often. The majority of cats don't experience any problem. However, when we do see a problem, it tends to be something like a really severe case of pneumonia, an abscess that won't go away despite aggressive therapy, persistent cystitis or some other "ordinary" illness that has managed to become a bigger problem due to the presence of immune suppression. For this reason, it is important to report all secondary illnesses to your vet when a corticosteroid product like DepoMedrol (Rx) (methylprednisilone) is being used on a chronic basis.

The other complication to be aware of is an increased tendency to develop diabetes in cats being treated with corticosteroids. If there are changes in appetite, even more urination than has been occurring due to the corticosteroid alone, weight loss, depression or poor hair coat, it is a good idea to check for diabetes.

Despite these complications there are several conditions in cats for which chronic use of corticosteroids is easily justified because the risk of not treating the conditions or the discomfort associated with them is clearly worse than the risk of using corticosteroids. Hopefully, someday, we'll have medications with the same benefits and fewer risks.

DepoMedrol can be successful for a very long time for many conditions. If the underlying disorder is progressive there will be a time when the disease symptoms can't be successfully alleviated but the length of time until that occurs can be very variable. We have treated a cat with intestinal lymphoma, identified by intestinal biopsy, for over four years, with DepoMedrol. This is a success story that is outside the bounds of what can be expected on a routine basis but it is an example of one patient who is doing well with very chronic use of this product. She has had one really severe bout with an abscess due to the immune suppression, though.

Hopefully, your Himalayan will experience similar success.

Mike Richards, DVM 1/5/2000

Methimazole -( Tapazole Rx) side effects -

Question: Thank you for your information about Tapazole and PLO cream and I understand your recommendation of this method of administration mainly for cats who are difficult to pill. However, doesn't it have the added benefit of not going though the stomach? I understand that many cats gets quite sick when taking Tapazole and I have also heard that Tapazole can cause kidney damage. Would the PLO method avoid these problems? Is Tapazole potentially harmful to the kidneys? Anne

Answer: Anne-

The side effects of methimazole (Tapazole Rx) are reported to be the same whether it is used transdermally, or orally, according to Dr. Alice Wolf, writing on the Veterinary Information Network. That is the only reference I can find. It does seem to make sense that GI signs might be less when it is administered in this manner, though.

The primary problems with methimazole are gastrointestinal irritation, liver disease and anemia due to adverse effects on the bone marrow. Of these effects the bone marrow suppression is the most severe when it occurs. Kidney disease is not a direct effect of methimazole but successful treatment of hyperthyroidism will sometimes cause problems in a cat that has pre-existing kidney failure. The kidneys try to induce high blood pressure to keep up blood flow when they are damage. Hyperthyroidism aids this process and seems to actually mask kidney disease in some cats. When these cats are successfully treated for their hyperthyroidism there is a drop in blood pressure. In a few cats with kidney failure this will be enough of a problem to cause a sudden worsening of the kidney failure they already had. It is important to monitor for this effect whenever treatment for hyperthyroidism is undertaken, regardless of the treatment method (methimazole, surgery, radioactive iodine therapy).

Mike Richards, DVM 12/26/2000

Corticosteriod side effects

Q: What are the side effects of Corticosteriods?

A: Carol-

Corticosteroids occur naturally in both dogs and cats. They are produced by the adrenal gland. They have effects on most of the body's systems. For this reason, using them therapeutically tends to produce a lot of side effects.

Cats are more resistant than dogs to the effects (and side-effects) of corticosteroids.

The most common side effects associated with the use of corticosteroids are increased drinking, urinating and appetite. These effects are less noticeable in cats than dogs but weight gain does seem to accompany the use of corticosteorids in cats pretty frequently.

Prednisone and other corticosteroids may cause other side effects, as well. Hairloss, dullness or thinning of the haircoat is occasionally seen in cats. Thinning of the skin and increased susceptibility to skin infections may occur. Panting is commonly seen in dogs as a side effect of corticosteroid use and is seen less frequently in cats, as well. We have had a couple of feline patients who developed diarrhea as a side effect of prednisone usage. Ulcers are reported to occur sometimes after corticosteroid usage but I don't recall seeing this problem in any of our patients.

Generalized immune suppression can occur, particularly at higher corticosteroid doses or with frequent administration of corticosteroids. This is the side effect that we worry the most about in cats, as there is some evidence that corticosteroids can make it possible for dormant feline leukemia virus or herpes virus infections to reappear. It is a good idea to watch carefully for signs of illness when it is necessary to use corticosteroids in a cat and to report any signs of illness to your vet as soon as possible.

Corticosteroid may lead to an increased incidence of pancreatitis in dogs but I am not sure if this problem occurs in cats. Use of cortisones makes it harder to control insulin dosages in diabetic pets and they may even help to induce diabetes in susceptible pets.

In people, corticosteroid induced pyschoses are reported. I honestly think that this occasionally occurs in pets but have no real scientific data to support that belief. Still, if your cat starts hiding out all day, is more aggressive, or exhibits any recognizable behavioral changes it would be worthwhile to report them to your vet.

There are probably other side effects that I haven't thought of. Sometimes when I look at the lists of things that corticosteroids do that aren't good for the patient it is hard to understand why we use them. But it is important to remember that they also have beneficial effects, as you have seen. As long as the beneficial effects are important and are providing an increased quality of life it seems reasonable to use corticosteroids. You just have to be careful to watch out for the pitfalls and adjust treatment plans accordingly.

Hope this helps.

Mike Richards, DVM 5/20/99

Ivormec/Ivermectin Toxicity in Cats

Q: Dear Dr. Mike, We have recently had two cats treated for ear mites with the injection of the above drug. One an 18 year old and one a 10 month old. Both cats became ill after being treated. The symptoms were the same in both cats being: ataxia, circling, depression, blindness, distress, staggering, falling over, twitching of the head and eyes. 18 year old 1st injection 1/26/99 - became ill 2/5/99 - 2nd injection 2/8/99 (had second injection as at the time had no suspicion that it was ivomec that may have caused his symptoms - Doctor thought bacteria from his teeth may have gone to the blood stream and caused stroke like symptoms - subsequently had some teeth removed) 10 month old 1st injection 1/23/99 - 2nd injection 2/10/99 - became ill 3/6/99. Then became suspicious as we had two cats in the same household with identical symptoms. In trying to pinpoint what may have caused both to become ill could come up with nothing that we had done differently except that we had the injections of ivomec for the mites. The veterinarian said that cats were not sensitive to this drug (some dog breeds), but not cats and that if it had been the drug they would have become ill much sooner after the injections. He felt that it may be something in the water (lead). I kept going back to the drug as a possible cause as the water just didn't make any sense as the 18 year old cat had been drinking this water for 5 years and is in perfect health, plus he has had some improvement since being home and drinking the same water. I then called the Animal Diagnostic Health Laboratory at MSU and spoke with a toxicologist there to get a second opinion. He too told me that some of the signs were consistent with the drug but not all and again cats were not sensitive to this drug. If there were the slightest chance that was the cause, he said it would be the first time he had ever heard of it. I thought okay, I've had two opinions and they are both the same, however still kept going back to the ivomec. That's when I got on the internet, found you, and also found some information contrary to what I had been told. You mentioned that it is not approved for cats, that it is an unlabeled drug (I'm not sure what that means exactly) and there had been some reports of toxic reactions in cats warranting caution when giving the drug by injection. I also have found reports from the Center for Veterinary Medicine showing there have been adverse reactions and from the Pet Action League that if used, the dosages are very small, not approved and the Vet should have informed us of this before proceeding. I am pretty well convinced now that this was the cause. What I would like to know from you is what can we do, if anything, to make our cats well again. We are beside ourselves as they mean as much to us as our children do. As I mentioned, there is a gradual improvement in the 18 year old, but I don't feel he will ever be 100% again. The 10 month old has only been home for 2 days and he still acts like he is drunk and very lethargic. I think maybe he has improved somewhat since last Saturday, but obviously he is not normal and something is wrong. Also, can you provide me with the source of your information that there is evidence of toxic reactions to this drug when used in cats? I would appreciate any advice and information you can give me at this point. Obviously there is some urgency in my request as we want to do whatever we can to get them well again. Your reply will be greatly appreciated. Thank you so much, Lois

A: Lois-

I have heard of reactions to ivermectin in cats primarily through the computer bulletin board systems for veterinarians, including the Veterinary Information Network.

We have not experienced a reaction to ivermectin in our practice and we have used it at times in cats for really difficult ear mite infestations in multiple cat households.

The symptoms you describe are very close to what we have seen in dogs with reaction to ivermectin and therefore, I think that it is likely that ivermectin could be the cause of the symptoms seen.

The treatment for ivermectin toxicity is supportive care while the patient recovers from the neurologic signs. While we have not treated a cat for this problem, all of the dogs we have treated have made full recoveries within two weeks. We have seen some incredibly high doses of ivermectin administered to dogs by people who were trying to use horse or cattle preparations of the product for heartworm control. While the toxic signs were frightening, they did resolve. I am hopeful that your cats will have the same outcome.

Mike Richards, DVM 3/14/99

Last edited 05/27/03


Michael Richards, D.V.M. co-owns a small animal general veterinary practice in rural tidewater Virginia. Dr. Richards graduated from Iowa State University's College of Veterinary Medicine in 1979, and has been in private practice ever since. Dr. Richards has been the director of the PetCare Forum...