Why you need to ask your vet about pet drug interactions & side effects

Discuss potential medication side effects with your vet

In the middle of a busy veterinary practice, many veterinarians may overlook an opportunity to discuss the possibilities of drug interactions and pet medication side effects. For example, many commonly prescribed nonsteroidal anti-inflammatory medications (NSAIDs) can sometimes cause digestive upset and/or liver/kidney problems in some sensitive pets. Corticosteroids like Prednisone can cause similar problems in some pets, as well as predispose pets to diabetes with prolonged use.

While a topic of debate, these risks are increased if Prednisone and nonsteroidal anti-inflammatory drugs are used together.

While potentially lifesaving, antibiotics can also cause allergic reactions from digestive upset to skin irritation. Some pets are sensitive to topical flea and tick medications, and in my practice I always administer topical flea and tick medication on a separate day from heartworm preventative medication or vaccinations, in order to lessen the burden on a pet’s immune system.

Whenever multiple drugs are prescribed it is always important for pet owners to ask questions, because veterinarians not only prescribe pet medication but are also dispensing these medications in private practice.

Do you have a question on potential pet medication interactions or side effects? Leave a comment below. 

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  1. My 108 lb Rottweiler has had skin allergies for the last 4 years which resulted in a lick problem. His front paw had a lick granuloma for almost 2.5 yrs. Endless refills of Baytril finally helped with the lesion but he still has that tendency to lick. He has now scratched an area on the side of his face, near his ear that is red ,bleeding and very sticky. He also is shaking his head, which makes me think it an ear infection? My vet prescribed cephalexin 500mg, 3 pills in the am: 2 pills in the pm. Can I also give him the Benadryl as he is still scratching the area?

    • You can try benadryl at dose of 50 to 75 mg twice daily to see if helps with itch. If not, you may need stronger antihistamine prescription antihistamine and/or low dose prednisone. You can also ask your vet about newer nonsteroid allergy options such as apoquel and cytopoint

  2. I want to know if there are any known contraindications between Trifexis and Zonisamide..my 11lb epileptic Schnauzer was just upped to 50mg of Zonisamide BID..up from 25mg BID..She takes Trifexis monthly..

    • No interaction between zonisamide and trifexis as far as I know specifically but check with local vet who prescribed it for confirmation. I would be aware, though of the rare potential for trifexis to aggravate seizure tendency in very rare dogs on occasion, in my experience and opinion, so make sure she is tolerating that drug ok, as well as check with local vet on their feelings on heartworm/flea meds

  3. Is there any interactions between Clavamox and Clopidogrel for felines….my daughters cat has URI and he has a heart problem so he was given both these meds at the same time…he is either having an allergic reaction to one of the drugs or they are interactions going on between the meds…please let me know ASAP….I am very worried about her cat and suggested she stop giving him the meds…he has a hard time breathing and twitches lying on his side for 5 to 6 hours, then gets up and eats, drinks, etc. but he still has a breathing issue….

    • There is no specific reactions between Clavamox and Clipridogrel as far as I know. However if breathing issues and/or twitching continues, then best to see vet ASAP, and of course stop both meds until the local vet who knows your pet best can revaluate the situation. Might be more related to heart disease itself, rather than either of these drugs.

  4. What are the contradictions in giving a dog with hypothyroidism Chlorpheniramine? I am caring for my sisters husky while she recoopperates from an illness and the dog was in bad shape. She had several bald spots and smelled of yeast. I have gotten her pretty much better but have been treating the severe itching with Chlorpheniramine. I just found an article that says not to give it to a hypothyroid dog. Why not?

  5. I myself am not familiar with chlorphenirimine contraindications in dogs with hypothyroid, however anything is possible. I would check with your local vet. In mean time, I would change to benadryl at dose of 1 mg per pound twice daily as option, as I have used benadryl in many hypothyroid dogs without issue

  6. My cat is on Phenobarbital for seizures. Would I be able to give her a Capstar for fleas? Thank you so much.

    • I would check with your local vet or even veterinary neurologist on whether ok to give the capstar in your pet with tendency to seizures, as occasionally capstar may trigger a seizure in occasional animal. Sometimes prednisone or dexamethasone can help lessen that possibility but best to check with local vet who knows your pet and history best

  7. Patricia DowningMarch 17, 2017 at 2:22 pm · Reply

    My 12 lb poodle Yorkie mix is on 50 mg twice per day of Zonisamide and insulin for diabetes. He showed elevated liver enzymes prior to everything prescribed . Concerned about potential side effects. His seizures did not last more than 5 minutes and com every month to 2 months. Diabetes was just confirmed 2 days ago.

    Other dog is 10lbs same breed and he does have elevated liver enzymes, but not diabetic. He’s also currently on an antibiotic after having teeth pulled on March 1.

  8. My 8 pound female Chihuahua who is approximately 7 or 8 years old appears to have an odd reaction when put on doxycycline and prednisone. Years ago, a vet recommended this as a soft kill for heartworm infestation and it worked…however randomly she would just stop doing whatever it was she was doing and yelp out in pain out of nowhere…we stuck it out to eliminate the heartworms, and she is now heartworm free.

    However she has developed a nasty cough for the last few months, and so the vet decided to use this combination again to treat the cough (suspected infection). This is a different vet from the first.

    The second night of the treatment she did the exact same thing…walked up to me sitting in the chair, lifted her leg and yelped out in pain, just like years ago.

    I don’t believe this is a coincidence as it has never occurred in between uses.

    Are there any know rare or uncommon interactions from these two drugs in small dogs? I’m at the point I don’t want to continue her on this particular treatment.

    • HI Brandon. Definitely an odd symptom you describe while on the combo of doxy and pred. As far as I know there is no documented reaction between these two drugs like this that would be responsible for this odd symptom, although you may want to see a veterinary internist or specialist who can review workup thus far and/or see if any other possibilities for causes might be present. From your description, it sounds more like a spinal and/or musculoskelatal reaction i.e disc or sore muscles which can be a problem on occasion as rare reactions to drugs, vaccinations, etc i.e a myosoitis(inflammation of muscle), or neuropathy…. Things to ask the vet specialist

  9. Can a dog that is taking simplicef take medrol at the same time

    • As far as I know there are no problems documented when taking simplicef and medrol at same time, but I would follow your local prescribing veterinarian’s recommendations on this, as well as have appropriate followups depending on what the problem is that is being treated

  10. My Doberman was diagnosed with valley fever and was started on fluconazole 250 mg BID then was found to have hypothyroidism and started with levothyroxine 800mcg, retested and raised to 1000mcg result came back 0.9 lower then last test, third test just came back as 1.0. His levels are not going up myself and our vet is perplexed. My other Doberman has same problem and his levels were normal after only 1 test. (Note the other one doesn’t have valley fever only hypothyroidism) anyway could the fluconazole be making the levothyroxine in effective because I give both meds at same time ?

    • I would first make sure that your vet has done a comprehensive thyroid profile to begin with at the start to make sure your dog actually has low thyroid. This includes not only a T4, but Free T4, TSH and thyroid and thyroglobulin autoantibodies. Many vets try and short cut their thyroid testing. But if your dog is truly hypothyroid, it will be very hard to see the T4 and even the Free T4 climb into the normal range until the Valley fever is brought under control. Unfortunately, other illnesses in the body may artificially lower thyroid levels on their own, in what is known as Euthyroid sick syndrome, so that measuring higher levels may not be possible until the Valley Fever situation is treated and fully addressed.

  11. My 12+ year old beagle mix girl is currently taking gabapentin, phenobarbital, piroxicam, and stasis breaker. She has a lung tumor in her left lung, spleen tumor, mass in her bladder, cyst on her left kidney and enlarged liver. She has not had any seizures in almost a year since she has been taking her medication on a consistent basis. I have not given her the heartworm preventative, Heartgard Plus, in the past 2 months. Will the Heartgard Plus interact with any of the above mentioned medications or harm her in anyway due to her medical conditions? She is a tough little dog. She amazes the veterinarian with how she continues to live pretty normal. She is not in pain, thank God, at this point.

    • Hi Katie. I personally would not give heartguard in pet on all of these drugs and with lung cancer, spleen/bladder tumor, etc. I would not use any chemical heartworm meds or flea/tick meds on pet like this, due to the risks of worsening the other diseases states through pressure on the liver/kidneys, etc. IF you want to use any heartworm and/or flea meds, I would try natural products such as the Bio Vance herbal heartworm protocol from http://www.fiestapetdeli.com

  12. I have a 6 year old female beagle that has cholangiohepatitis and is taking Clavamox, ursodiol, enroflaxcin, and denamarin. Can any or all of these medications make her constipated? Thank you.


    • These medicines usually dont make dogs constipated, however, dogs with liver disease are prone to stagnant blood flow in the liver and subsequent constipation. Ask your vet if ok to add a teaspoon or two of soluble fiber like metatmucil to meals along with a teaspoon of wheat bran as insoluble fiber as well

  13. My vet wants me to give my 4 year old; 15 lbs. alpha lapso 150mg twice a day of rilexine but im afraid to give it to her because of the serious side effects that it can have from vomiting, diaherra, lameness in back legs, even death.. what do you think of this medication?

    • This generic form of the antibiotic cephalexin is usually quite safe in the vast majority of animals. As with any antibiotic, or medicine for that matter, very rare side effects may be seen. The most common one with this drug is self limiting diarrhea and/or vomiting, which usually resolves quickly on stopping the drug, but this side effect is not common and is even less common when the drug is given with food. I would check with your local veterinarian who prescribed the medication if you have any questions and/or concerns.

  14. my dog takes 100 mg of theophylline 3 times a day and has famotidine 20 mg twice a day. are there drug interactions between those for dogs the same as with a person?

  15. i dont see my question so here it is. my dog is on 100 mg theophylline xr 3 times a day and i give him 20 mg of famotidine with breakfast and with lunch. is there a drug interaction between those that i have to be worried about. i see on drugs.com there is a moderate interaction in humans.

    • HI Faith. I have not seen any reactions in my practice on the few rare occasions I recall prescribing these drugs together i.e theophylline and famotidine. Of course, theophylline and/or any other prescription drugs for canine patients should always be monitored by the local veterinarian who prescribed them.

  16. Hi there. My dog is taking 7.5 mg of Piroxicam SID and he is extremely itchy. I’ve had him on Apoquel but it is too expensive to continue although it worked wonders. I’m wondering if it is ok to give him Loratadine to prevent the itching? Any contraindications?

    • While I dont have experience using Loratadine in veterinary practice, I dont see a problem with over the counter antihistamines and piroxicam, although I would check with your local vet who knows your pet best. I have more commonly used meds like Zyrtec or old fashioned benadryl as over the counter antihistamines.

  17. Can pyrantel betaken with methocarbamol?

  18. Can my dog take pyrantel while on robaxin 500 mg

  19. Are there any contraindications with Zyrtec & Atopica. My cats been on atopica for like 8 months & it’s not working. I read that Zyrtec was a good medicine for his itching, biting, over grooming, runny eyes & sneezing. I know the Zyrtec is just to deal with the symptoms, that atopica can help in the long run. I’d like to keep him on both if there are no contraindications. Thank You!!!

    • HI Lisa. As far as I know there are no known contraindications with Atopica and Zyrtec. I would let your local veterinarian know that you are adding the Zyrtec on board. I would also ask your vet about newer prescription allergy meds which are even safer than atopica, at least in short term, in apoquel and Cytopoint injection; both of these latter meds are by prescription as well.

  20. Jennifer L McmicanApril 19, 2017 at 12:19 am · Reply

    Is it okay for a dog taking prozac to also use dasuquin?

  21. Can my 7 year old westie take apoquel for itching while taking confortis for flea control?

  22. Hi Michael! My 8lb dog is taking 62.5mg twice per day of metronidazole for diarrhea and is now acting like she has fleas. She has been flinging her head around so as to try to catch something on the back of her head or ears and then also biting near her tail and under her back legs. I don’t see any bites or fleas but I’m assuming that’s the problem. Is it safe to apply her Frontline Plus to her while she is taking the antibiotic? I don’t want to overload her with too many meds at once. Thank you!

  23. My 4 yr. old American pitbull is on apoquel for her itching issues , she now has diarrhea and I was going to try some liquid Pepto bismol to see if that will help her , along with chicken broth and white rice. Im wanting to know if anyone else has used this combination of medications and if so, did your dog have any kind of bad reaction to the two.

  24. Hello, is it ok to give my cat 10 mg prednisolone (that’s her daily dose), gabapentin for travel (we will be moving on this day, driving in a car for 6 hours), comfortis flea pill 270 mg, mirtazapine 7.5 mg compounded cream in ear flap. My cat has IBD. I would give her the mirtazapine, comfortis, and prednisolone about 4 days before the move. And then on moving day just give her prednisolone and gabapentin. But on moving day she will still be affected by the mirtazapine and comfortis because they both last longer than 4 days. So she will be affected by all 4 meds on moving day even though she will only be taking prednisolone and gabapentin on moving day.

    I’m desperate for this info. Thanks very much.

  25. My 14 year old dog is on rimadyl was wondering if giving him confortis at the same time is a good idea or not thank you for any information you can give

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