Medication Questions


Medication Questions

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 The ITP is almost always reversible if the medication is withdrawn.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

Assessing valid scientific study

Info on Prozac

On the page that discusses Prozac, Dr. Mike says he's not familiar with any studies done...I'm surprized he's not aware that most if not all medications that deal with brain chemistry are first tested on dogs before given to humans. Secondly, I can't believe he hasn't heard of Tuffs University?? They're focus is on behavioral drugs for dogs...Prozac, in particular, is one of THE most tested. I believe there have actually been thousands of tests, world wide, on the use of Prozac and dogs. If you're interested in more info on Tuffs Univ. Let me know and I'll get the number for you from my vet...I have consulted with them in the past and was very impressed. Thanks! Katie

A: Katie- I think that there is some confusion about what I mean by a valid scientific study and what many people perceive to be a valid scientific study. There is also a problem with our website, in that we did not start dating the postings until this year. It makes it hard to evaluate which information is newer on the site. There were two studies published in the fall of '98 on the use of fluoxetine (Prozac Rx) in dogs that are valid controlled studies. While it may be true that many medications that are intended for use in people are tested in dogs I don't think that anything close to the majority of testing is done in dogs. At the PubMed site, with the National Institute of Health's medical journal database online, there are only 9 references found on a search using "fluoxetine and canine". Of these nine references, only two deal specifically with disorders likely to be treated by veterinarians in practice and one of these studies only involved nine dogs. The other study involved 42 dogs. Clinical trials are most reliable when a medication is compared against a placebo and neither the patient nor the practitioner know which is being used in any particular patient. These studies are known as "double blind" studies. It is reasonable in some cases to do "single blind" studies, in which the practitioner knows whether the medication is a placebo or the real thing but there is still sometimes inadvertent prejudice in single blind studies. Good double blind studies are much less common than other forms of "scientific reports". Scientific studies may be done for lots of reasons other than clinical use of a medication. While these are real "scientific studies" and may be useful in the long run, they may not be helpful in determining if there is a valid clinical use for a medication. Case reports are the most unreliable of studies. Just to give you one example, which I wish I could go into more detail on, I once treated a dog for a condition that had the exact same medication and dosing directions in several textbooks, all of which insinuated that there was a "standard" treatment. Out of curiosity, I researched the references listed, since there was a short list of three or four references in all the textbooks. After looking into these references I was able to discern that ALL of the information presented was based on ONE clinical case report in which a dog was treated with a medication and recovered. Since I didn't want to use the "standard" treatment, I didn't do anything. The dog recovered uneventfully. If I had written the first paper, the "standard" treatment would have been to do nothing. Both "treatments" worked but neither was tested in a scientifically valid manner. It is critically important to evaluate the validity of information from all published sources, including this one! I am familiar with Tufts University. At the present time I am not sure who the behaviorists are who are associated with the vet school there but I think that if you ask them they would agree that there are limited numbers of scientifically valid studies done on the clinical use of fluoxetine for any particular behavioral disorder in dogs -- the only ones I know of being a small study on aggression, several on cataplexy (a very rare condition) and a larger study on lick granulomas. I hope that this makes my statement a little more clear to you and to other readers of this site. I will have to try to come up with a better way to explain when there is information available that has been properly validated and when the information is less valid for clinical situations. Mike Richards, DVM 4/5/99

Long Term Medication

Q: Hi Dr. Mike! I have a 4 year old male Rhodesian Ridgeback who developed food allergies at about 4 months of age. We took him to a university vet school and saw two dermatologists. After blood tests, skin tests, and trials with several diets, we have found a regimen that appears to keep my dog's allergies in check. First, we have, through trial and error, found that he is allergic to chicken, beef, pork, eggs, and Hill's CD cat food (which I feed my cats). Second, his symptoms include: Within 2 days of the food violation, he begins to lick and bite his knees, then welts appear on his back, oozy sores appear on his underside, and he develops yeast infections between his toes and in the folds of his mouth, also his ears get infected. If I see him eat something he is allergic to, I give him 3 prednisone immediately, which seems to ameliorate the subsequent outbreak of symptoms. The regimen that we have found works best is to feed him 1 can of Innovative Veterinary Diets, Limited Diets (venison & potato) and 2 cups of the same, but in dry kibble form daily (this is divided into 2 meals -- morning & evening). Every other week we give him 1250 mg Cephalexin twice a day, along with 3 benedryl caplets twice a day. When he has symptoms we also give him prednisone as needed, panalog for his ears, and Conofite lotion 1% (Miconazole nitrate) for his yeast infections. I am concerned that these medications taken on a long-term basis may cause some type of damage, such as liver damage. In your opinion, does the above regimen sound reasonable? Do you know of any side effects or long-term effects the above drugs may have on my dog? Thanks so much for your advice! I really enjoy your web site! Anita

A: Dear Anita I do not know of long term effects associated with the use of diphenhydramine (Benedryl TM), cephalexin (Keflex Rx), or miconazole ointment (Conofite Rx). Cephalexin, an antibiotic, does sometimes have side effects but these would probably show up early in the use of the medication. We have used cephalexin daily to control persistent skin infections in several dogs. We kept one dog on this medication daily for about three years because he had severe problems whenever we withdrew the medication and did well on it. We are into the second year of use in another dog currently, with no problems. While this is a limited number of cases I still feel pretty comfortable with long term use of cephalexin when it is indicated. Prednisone can cause problems (see the other note posted this week) with long term use. It is very important to use the lowest possible dose and to give it no more frequently than once every 48 hours. Longer intervals are even better when possible. Some vets do like to "pulse" prednisone, giving it daily for 5 days and then not giving it again for at least a week or so, then repeating this cycle as necessary. As far as I know that is OK but I am more comfortable with this approach when I am getting at least two or three weeks between the need for prednisone pulsing than I am with shorter intervals. Prednisone will sometimes cause liver damage but this appears to be an unpredictable reaction that isn't necessarily related to chronic use. Mike Richards, DVM


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...