Onsior for cat pain (info on side effects)

At the time this article was first published (2013) Onsior was a fairly new painkiller for pets. The active drug is robenacoxib. Before this drug became available in June 2012, a cat owner had little else to choose from when their cat was in need of drug therapy. You had Metacam, which has proven to be at the least unpredictable and at the worst deadly. Or you had Buprenorphine, a drug 30 times more powerful than morphine and in the opiate family.

Onsior painkiller for cats
Onsior painkiller for cats. Image: MikeB
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Onsior say

Onsior has a website that tells us more about the drug, which is classified as a NSAID (nonsteroidal anti-inflammatory drugs). It’s similar to Meloxicam and Celebrex used for pain management in people. Their company webpage states:

“Pain delays recovery, impacts negatively on a patient’s well-being, and disturbs the bond with its owner and also the veterinary team.”

Discussion and side-effects

The small, yeast-flavored 6 mg. tablets provide a full 24 hours of pain relief. It’s primary uses are for postoperative pain associated with orthopedic surgery, hysterectomy and castration in cats. Onsior reaches maximum concentration in only 30 minutes via the intestinal tract, and is eliminated from the blood within two to four hours via the biliary route.

Onsior is recommended only for three days. It’s usually given half an hour prior to surgery, then 24 hours later the cat gets a second dose, followed by the final dose 24 hours after the second dose. The drug is sold in a blister-pack holding three pills. The pills are around $3 each. For cats over 13 1/2 pounds, two tablets daily are required.

In a field study, the most commonly reported adverse reactions were surgical site bleeding, infected surgery sites, lethargy, vomiting, and appetite reduction. Onsior should not be used in cats that have a hypersensitivity to robenacoxib or known intolerance to NSAIDs. It also can’t be administered with any other NSAID or with corticosteroids.

The most common side effects are lethargy, vomiting, loss of appetite and increased bleeding time. Clinical studies on cats dosed up to 5 to 10 times the daily dose for 6 weeks to 6 months, reported no deaths and/or encountered clinical side effects resolved upon termination of medication.

Update on side effects as at 2021: The Onsior website says that this drug is safe and efficient for use up to a maximum of three days. The report that the most commonly reported adverse reactions were: surgical site bleeding, infected surgical sites, lethargy and loss of appetite. Other possible side-effects include vomiting, cystitis, haematuria, diarrhoea, hair loss, bruising, excoriation, cardiac arrest, ataxia, weakness. The chart below shows the prevalence of these side effects and a study conducted by the manufacturers of this drug. It is published online on their website.

Lorie Huston DVM – input

Lorie Huston DVM on Ingrid King’s website writes that as a 2011 Onsior had similar problems to Metacam in terms of kidney damage. She says that veterinarians at that time were cautiously optimistic but were fearful of the side effects. At that time Onsior was used more often in Europe than in the US. She also states that there were less side effects for Metacam when used in Europe than in the US. It was unclear as to why. The possibility is that veterinarians in the US use the drug inappropriately compared to that in Europe because they haven’t limited access to the proper educational materials about the drug.

VCA Hospitals – input

A large animal hospital chain, VCA hospitals, say (Rania Gollakner, BS DVM, MPH) that some parents are hypersensitive to robenacoxid and other NSAIDs and if so this drug can have the side effects mentioned above. It should not be used they save for patients who are dehydrated or have renal, heart or liver disease. Should not be administered with any other NSAID or corticosteroid. Pets should be monitored closely when given the drug. The administration of the drug should be stopped immediately if there are any adverse effects.

Chart on prevalence of side effects from Onsior field trial

Onsior side effects
Onsior side effects – prevalence as per a field study as published on the Onsior website.

Elisa’s comment

I read several reviews where Onsior was used to treat arthritis in cats. This confused me, because the drug is marketed for short term use for mild to moderate pain. I don’t know how long-term use would eventually affect a cat. Especially since it takes years for some drug companies to warn the public about negative consequences. NSAIDS are notorious for causing gastrointestinal upset. Hopefully it won’t cause something medical to pop up five or ten years down the road.

Having rescued several injured cats, we’ve used all three drugs for pain management. Although Metacam didn’t cause any side effects, I was on edge after reading up on its negative reviews. Update December 1, 2021: since his article was written by Eliza in 2013 it’s come to my notice that the drug in Metacam, meloxicam, is under an FDA (Food and Drug Administration) warning online. They say in clear terms that repeated use of meloxicam and cats has been linked to acute renal failure and death. It’s been approved for one-time use in cats before surgery to control post-operative pain and inflammation. From my personal experience, I can say that I dislike this painkiller. I gave it to my now deceased male cat who had cancer behind his right eye to relieve pain and I think that it caused kidney damage to my horror. It hastened his death through euthanasia. I still regret approving its use.

Our cat Sealy was on liquid Buprenorphine after his two operations. Since it’s a narcotic, it left him quite drugged. Other than that, he experienced no side effects. He sat in his cage and stared at us, or he ate. It was clear he was on strong medication. Sealy was on the drug for several weeks, and we “weaned” him off of it, being an addictive opiate.

The Onsior was definitely the clear winner, used after spaying. No side effects at all and they drug appeared to do what it claimed it could do-reduce pain. No “zombie cat” from using this drug. It will be the drug of choice, should any of our cats require short term pain medication in the future.

Have any of the readers used Onsior? I’m curious whether it’s been prescribed for long-term use for pain. It would be rather expensive, should a cat need to stay on it for longer than the three-day recommendation.



  1. http://www.theuniversityanimalclinic.com/
  2. http://us.onsior.com/
  3. Updates by MikeB at Dec 1, 2021

Note: this page was first written in 2013 and upgraded and re-published on Dec 1 2021.

28 thoughts on “Onsior for cat pain (info on side effects)”

  1. My dear sweet cat Magneto has mammary cancer *tumors along the entire mammary glands on her right side, one is substantial and it is open and oozing. She is on an antibiotic Clavaseptin 21 pills- 1 tablet every 24 hours. She is failing and there is no help for her. She is terminal and at a time when it becomes more obvious that she is sick and uncomfortable I will do the most humane thing for her . She is using her litter box less and less, only eating a freckle of food and sleeping a lot. She does not appear to be in pain, but I understand that cats hide their pain. The Vet recommended Onsior. I just gave her one. From everything I’ve read in my research on this drug…it says for 3 days only. The vet has given me 16 pills. How cautious should I be with this. I’m trying to make the time she has left on this earth as comfortable as possible for her. I love her so. My heart is broken.

    • My dear, I feel your pain and distress. Don’t misunderstand me but I think the best thing might be to not delay anymore and let her go. Be careful that you don’t hang on too long. I did that once and I regret it. Only from what you say I am not convinced that Onsior is the answer. What if there is an adverse reaction? That means more suffering. These are just my personal thoughts. Only you can decide. What did the vet say about euthanasia? The very best of luck.

      • The Vet said that once she becomes sick I should have her euthanized. Since last I emailed I have given Magneto onsior twice (the dose is 1 daily) I gave her one at noon on Saturday and again noon Sunday. I am also feeding her, her very favorite wet food to see if it would encourage her to eat. For the last 2 days she has eaten a bit and in fact I was really surprised to take 3 pee clumps out of her litter box this morning and 1 poop (she hadn’t pooped for 4 days and didn’t pee for 2 days). I did not give her an onsior today (Monday). If she was going to have an adverse reaction to Onsior would it be immediate? I’m really beside myself with grief, fear, sorrow. She doesn’t appear to be in pain at least not yet…she isn’t well I know that and she is sleeping a lot but she doesn’t seem to be terribly uncomfortable. I do not want her to be suffering but I do not want her to go until I know she is suffering. The vet didn’t give me anything for her pain when I was there. I called the following day and asked if I could get something (to have on hand) for her if I felt she was in pain. Originally, when the Vet told me she was pretty certain it was advanced mammary cancer, she said that the only way to know for sure would be a biopsy. That is when I came home researched and researched on the internet and concluded and confirmed in my own mind that the vet’s suspicion is right. She said after confirming by biopsy she would not recommend a mastectomy because it is extremely painful and the odds of survival are very low. That is when I decided not to have the biopsy. Magneto stresses and is anxious at the sight of the carrier and further stresses at the vet’s office so I didn’t want to do that to her. In fact I asked the vet, that when the times comes to let her go I did not want to take her to the clinic. I just cannot stand the thought of having her stressed and full of anxiety knowing I would be taking her to be euthanized. l will pay extra for the vet to come to my/her home to have it done. Are there other pain medications. Thank you for your kindness in responding to me. I’m a mess… Heather

        • Contact a cat rescue you trust and ask them what they’d recommend for pain. Your kitty would still have to see a vet to get the meds but you could mention whatever drug the rescue tells you to use. I’m so sorry you’re going through this.

  2. My 13 year old tuxedo cat Sassy is on Onsior. The oncologist that I take her to suggested that we try it to manage her pain. She was diagnosed with bone cancer in her jaw. She’s been on it for almost one month and her blood work came back good. The oncologist said it was “remarkable”. I feel very lucky to have this medicine for her so I can enjoy a little more time with her.

  3. Thank you so much for posting this article! I am sorry I am so late in finding it. There is precious little information about off label use of so many drugs. She is a magnificently beautiful cat, inside and out. Still cannot believe someone threw her out. But their loss was our gain. Anyway, above all, the cat’s quality of life should be consideration #1. We are continuing the every third day dose until we cannot. Thank you for your site and warm purrs to all the wonderful lovers of cats present here.

  4. Hello, Our 16 year old dilute calico spay rescue suffers from severe degenerative arthritis and stomatitis. She is extremely sensitive to medications especially antihistamines and antibiotics. During a recent flare up, she was prescribed Buprenex (not the first time) which was only helpful for the first day. She has not done well with any pain management until our vet prescribed Onsior for a 3 day trial. She became a different cat. Relaxed, present, happy and able to comfortably eat. Talking. Able to reasonably ambulate the many cat stairs in the house. Able to walk from one end of the house to the other without having to rest on many of the dozens of rugs laid out to aid her. So, beginning on 9/13, starting with the original three day dose, we did two weeks of every other day, then two weeks of every third day. We tried pushing it to every 4 days, but the stabbing mouth pain she experiences when eating reappeared. All of this was done with our vet’s consent and we will do regular blood tests to monitor as we do with all of our feline companions. We know there are risks but we would rather have her enjoy a quality (possibly) shorter life than a (certainly) pain filled longer one. She is also on Cosequin and a grain free, high quality protein diet. And tons of love.

    • Thanks Grace for a very usual comment which adds to the article. Very useful and most welcome. And what a fine photo of a beautiful cat. So sad that she has this pain. Well done in helping her.

  5. I have had my 13 year old cat on Onsior to help with her joint problems for a couple of months now, without noticing any side effects. However, I did notice quite a difference once she started taking a natural supplement called Seraquin. We’ve got our cat back now! She cat sit upright, she can run and play and even jumps onto the shed on a good day. Hope this helps 🙂

      • Hi everyone. I thought I’d chime in, too, since my baby guy has been battling arthritis the last 2 months. He is 16 or 17 (I found him as a baby when I was a teenager but not sure which age). Needless to say we are super duper close, as you all know! I feel terrible that I just noticed he was having trouble lying down and jumping recently, as it seems like it came out of nowhere but as I keep hearing from pet parents like yourself and my vet, cats are very good at hiding pain till it is out of hand. So, I got him to the vet who gave me Adequan (glucosamine/chondroiton) and Onsior. But not in that order. We first tried Adequan for a month but no improvement so I brought him back in for x-rays as I could tell he was having trouble still. I guess he got sore after the x-ray and started crouching while he was eating and drinking (his bottom wouldn’t touch the floor) so I got freaked out that something happened during the x-ray (but know it didn’t he just really got sore) so that is when they gave me Onsior. Now, my vet said that although he can take the 6mg table for 3 days in a row that a lot of vets give it for 6 days in a row. BUT, this morning he puked a little (maybe hairball trying to come up but now I am not sure) and I am thinking I should just stop for now (he had the 3 day dosage). Like you all have noticed, the change is REMARKABLE! He is doing SO much better now and of course, I am elated. I have researched like you all have and so far have to agree with the group consensus, it seems that the positives are outweighing the negatives. I will call my vet tomorrow (Monday) and see what she says but I think I feel most comfortable with 3 day dose for now, especially since his belly got upset today.

        What I am most interested in seeing at this point is if his pain will come right back to where it was in a week :/ I guess I’ll cross that bridge when we get there. Lastly, I have been sprinkling turmeric, ginger, omega 3 oil, and some msm-glucosamine-chondroiton (all in 1) capsule on his food for the last week. I just do a little because he was on to me and wouldn’t touch it, of course, but I have dome a lot of research before doing this and asked a couple of vets and it is VERY safe for those things to be administered to him. Maybe some of you might want to try it?

        Thanks for letting me share and I appreciate any feedback and also hope this may have helped others!

        • Hello Elizabeth. Thank you very much for your extensive comment which is very useful and which I enjoyed reading. It’ll certainly help other people who are researching this matter. It is much appreciated and I’m very impressed with what you are doing for your beloved cat. I don’t think anyone could do more or do better.

  6. I am very interested in trying Onsior for longer term, on my 19y.o arthritic cat. A couple of weeks ago, her condition worsened to some pretty serious lameness. We used Onsior for 3 days thanks to our terrific Vet, and Tillie’s bounce-back was really remarkable, particularly helpful with back leg/hip weakness. Now, 2+ weeks after the first spell, she is seriously lame again. My vet is open to the idea of trying Onsior longer term — either as needed or maybe one treatment every week or two. So I’m interested in hearing more about off label dosing in these kitties. My main goal is to make her more comfortable, and she had no digestive upset that we could see from the initial usage. She’s old, so balancing quality of life off against long term kidney or whatever function is what we’re after. Any thoughts?

    • Hi Patty, thanks for visiting and asking. To be honest I don’t know the answer for sure. However, the information provided states clearly that 3 days is the max and there are some potential considerable side effects even when used as stated. I have a gut feeling that this pain killer should not be used in the long term. And that is based on a risk/reward basis.

      There may be a middle ground in administering a low dose for a long time which may benefit your cat while minimising potential side effects. Sorry I can’t be helpful. Best of luck to you both.


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