PetMeds® Comfortis Flea Control: Potential Side Effects

Did you know that not all flea control products for dogs are applied to the skin? Many pet owners prefer oral administration of external parasiticides to avoid exposure of themselves to topical pesticides, which is why some dog owners prefer the ease of administering a chewable tablet such as Comfortis. Comfortis can control fleas up to one month on dogs, but it does not kill ticks

Comfortis kills fleas within 30 minutes and lasts for up to one month. In my experience common client concerns include lack of efficacy against other external parasites such as ticks, as well as the need for written prescription from the veterinarian.

One of my concerns with Comfortis is the occasional wide range of side effects including vomiting and/or diarrhea and various neurological reactions ranging from incoordination, trembling, and seizures.  It’s for these reasons that I generally prefer using other flea and tick products in my practice such as Frontline Plus or Advantage II.

However, clients do report mostly high satisfaction with this product, with the exception of these occasionally severe side effects.  It is also important to still treat the environment for fleas, as well as incorporate additional measure to help control other parasites such as ticks if using Comfortis.

Related Posts


  1. when in southern california, vet prescribed Comfortis for 10 lb and 15 lb chihuahua mixes. i was amazed how much they scratched soon after application, and was told it was the fleas coming out of their skin… already dead… and that this would kill adults and larvae and eggs alike. i put the pill at the back of their tongues and held their muzzles shut until i knew they’d swallow it… pretty easy with the learned technique, and very clean.
    … since then have moved to the mountains of arizona, where vet prescribes topical Frontline. i’m not sold on this product. just returned from visit to the beach in so cal, and came home with poor itchy pup. an oatmeal bath has not eased his discomfort.
    … it’s been only 2 or 3 weeks since last application of Frontline. i’m here to order Comfortis once again. i’ve never experienced side effects mentioned by others (my sincere sympathy), and have visible results within hours of dose… even though it’s a bit disconcerting to see them scratch like crazy for a while.

  2. Dr. Michael Dym, VMD veterinarianMarch 17, 2014 at 6:05 pm

    I dont have problem with comfortis in most cases. On occasion side effects can be seen with any pesticide. You will need written script from your vet to order the comforts

  3. I have an Epileptic 3 year old Aussie whom I can not give him any flea meds that contain Comfortis. He has never had a Flea before but lately I have noticed a couple and although I am quick to catch them, I know the risks & dangers of having even 1 tiny flea. I have been looking everywhere online, in pet stores as well as asking my VET. and I can’t seem to find a resolution. I need help finding a natural flea product where it won’t cause my baby to go into seizures every month. Can anyone Help please? Thanks

  4. Dr. Michael Dym, VMD veterinarianApril 26, 2014 at 3:58 pm

    Consider working with a holistic veterinarian on improving your dog’s overall health so not so attractive to fleas. See my website for more information. Many holistic vets do offer phone consultations. An excellent natural flea product is

  5. i had no adverse reactions and much less scratching after this mid-april dose of comfortis. i still prefer it to frontline. will report back if any bad experiences… otherwise, i’m sticking with comfortis because it seems to work great for my pups.

  6. Dr. Michael Dym, VMD veterinarianApril 28, 2014 at 2:47 pm

    Thanks for sharing

  7. Add me to the list of having a dog who quite likely had an adverse reaction to Comfortis. My 13 year old Border Collie had severe vomiting and tremors approximately 5 days after receiving Comfortis. The tremors became uncontrollable and progressed to the point where she could not walk. We really thought we were going to lose her. After rushing her to the ER to be stabilized and monitored, along with a follow-up consult with a neurologist, we were unable to make a positive dx. The one new factor in her life was Comfortis. After taking her off Comfortis, she is now completely normal 3 months post the tremors/neurologic episode. Was I informed by the neurologist of a possible connection to Comfortis? Nope, he only wanted to do a spinal tap and an MIR….Positive conclusion? Nope. Will I ever give her Comfortis again. H NO!!!

  8. Pingback:

  9. We gave our lil 20 lb dog comfortis two days ago shes normally very active shes only 4 yrs old. She slept and layed around for two days!!!! She had a bad odor coming from her skin we are NEVER giving her this again!!!!!

  10. She was also trembling.

  11. Dr. Michael Dym, VMD veterinarianJuly 4, 2014 at 6:31 pm

    Sorry to hear about this Mandy. Hope she feels better soon.

  12. I gave both my dogs a cap star….then bathed them a few hours later, then before bed I gave them both a comfortis. 6 hours later the little one is fine! The larger female has been out twice and acting like she can not get comfortable. Has this can’t get comfortable feeling for over one hour. Plus during her time of going outside she has had 3 bowel movements. She is just not acting right.

  13. Dr. Michael Dym, VMD veterinarianSeptember 24, 2014 at 10:34 am

    Could be reaction to too many meds at once. She should be fine in a few hours. If symptoms persist or worsen, see vet for evaluation

  14. I gave my 2 yo dog comfortis 2 weeks ago. After giving it to her I noticed she was having a difficult time walking, even became paralyzed for a couple hrs. She normally is very active and intergetic. It’s been 2 weeks and she’s still having difficulty getting around. I have taken her to the vet but they brush it off as being a spinal issue and tell me comfortis won’t cause these complaints. know this is a side effect, but was wondering if she will always have trouble now, or if maybe this will wear off with time.

  15. Dr. Michael Dym, VMD veterinarianOctober 14, 2014 at 3:59 pm

    Hi Tia. Hopefully it will wear off with time. I do agree with you, any topical or oral flea/tick pesticide may cause neurological issues in some sensitive patients. I would consider consulting with a holistic vet. to learn more about homeopathy see the booklet on the website and my website . Many homeopathic vets do offer phone consultations.

  16. I used comfortis on my golden puppy 4 times starting from 16 weeks old. The first 3 times he was absolutely fine. Last time he got the pill was 5 days ago. The next morning he started vomiting with yellow liquid, he stopped moving, eat and drink, we took him to the vet the next day, they made fluids, ton of injections and gave medication for using at home. He felt better right after procedure but he still didn’t drink or eat and started throw up in the evening. We took him to the vet again, they did the same and the result was the same. He’s continuing vomiting and refusing eat and drink. I don’t know what to do, it’s 4th day without food, so I literally push can food in his throat, and in couple ours he throws up this food and medication. Veterinarian obviously can’t help, I know for sure that it’s happening because of comfortis cause that was the only difference in his routin. Please help me somebody who had this problem! What else can I do to save my puppy? He’s dying in from me

  17. Dr. Michael Dym, VMD veterinarianOctober 19, 2014 at 3:16 pm

    There may be another condition in a young puppy like this such as an undiagnosed foreign body. I would ask your vet to send the x rays out for a board certified radiologist to read. Barium swallow x ray studies may also be helpful here. If not already done, I would recommend a FULL CBC/chemistry blood profile to make sure nothing else going on. If your vet stuck, then referral to vet internist in your area or specialist may be worthwhile.

  18. DO NOT GIVE your animals Comfortis or Trifexis (which has Comfortis). My dog ended up in the ER and had to be put on oxygen. If i had not taken him, he would have DIED over this medication! He could not breathe and was trembling. We took him to the ER and he passed out on the table and had to be rushed to the back for oxygen and add’l meds to save him. Wish I had read reviews before giving it to my dog! Look at all sites for all bad reviews on this medication and choose something else! Your dog or cat will love you for it!!

  19. If cat only threw up over an hr after ingestion will it still work? I hope some got into his blood. He itched like crazy for an hr or two after ingestion and maybe longer. Then some time during our sleep he threw up lots of kibble and fur. This morning he itched and shook once but seems settled. I am really hoping the vomit was from the sensation he felt after it was in his blood and NOT that the pricey pill was lost in the vomit on the floor! Expensive flea collar, Revolution, nor FrontLine worked.

  20. Our Shih-tzu had a reaction to topical flea meds, so vet prescribed a prescription flea spray that we used for 5 years. Then we moved to a hot climate and she got hook worms, so the vet put her on Comfortis. 6 months later at her regular 6 month check up, she now has liver disease! I dont think this is a coincidence. Nothing else has changed… we will be going back to the spray and now have to feed her special food and vitamins. 🙁

Leave a Comment