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Robenacoxib (Onsior)
Revised: February 01, 2023
Published: July 09, 2012

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Onsior

Available in 6 mg tablets for cats, and 10 mg, 20 mg, and 40 mg tablets for dogs


Robenacoxib is a member of the coxib class of non-steroidal anti-inflammatory pain relievers (NSAIDs). It is used to relieve pain associated with inflammation and is approved for three days of use after surgical procedures in dogs and cats. There are some special safety features for robenacoxib that are not shared by other NSAIDs, which have made this product particularly of interest for use in cats. This product was first approved for cats; the canine approval came later.

Feline pain relief has long been problematic with medications either being overly sedating, ineffective, or toxic. There has been a revolution in pain relievers for dogs since the mid-1990s but cats have been somewhat left behind. To this day, an approved anti-inflammatory pain reliever for long-term feline use is still unavailable. That said, while approved for only three days of use (six days in the U.K.), robenacoxib is commonly used more long term at reduced dosages with good results.

As mentioned, robenacoxib is an NSAID of the coxib class. Other members of the coxib class with which you might be familiar are Celebrex® (celecoxib - for humans), Vioxx (rofecoxib - for humans), and Deramaxx (deracoxib - for dogs). What makes this class of NSAID special is its preferential suppression of an enzyme called COX-2 (cyclo-oxygenase 2).

Inflammatory biochemicals responsible for the pain and inflammation we want to alleviate are produced by COX-2. One of the goals of pain relief is to inhibit COX-2 without inhibiting its counterpart COX-1.  Cyclo-oxygenase 1, abbreviated COX-1, is what is called a constitutive enzyme. This means it is involved in producing regulatory biochemicals (called prostaglandins) which are important in maintaining the normal health and function of our bodies. We want to leave this enzyme alone. COX-2 is induced in times of inflammation and mostly produces inflammatory biochemicals. We would like to suppress this and curtail the painful inflammatory process.

In the past, NSAIDs could not distinguish the COX-enzymes; they inhibited them both. With the development of COX-preferential and COX-selective NSAIDs, we can inhibit COX-2 and leave COX-1 alone. The introduction of COX-2 preferential NSAIDs has reduced stomach and intestinal side effects by 50% in humans and has made FDA approval of certain NSAIDs possible for pets. The coxib class of NSAIDs is COX-selective.

Robenacoxib concentrates at the inflammation site rather than simply circulating in the bloodstream. This mechanism affords an extra safety measure because the drug is sequestered away from healthy organs where its effects are not needed and focused on damaged tissues where it is needed.

How This Medication Is Used

This medication is approved for three days of consecutive use in dogs and cats. The patient, whether a dog or cat, must be at least four months old and weigh at least 5.5 lbs. If use other than what is approved on the official label is considered, monitoring blood tests and pre-treatment blood tests should be evaluated.

Side Effects

Drug side effects are classified into two types: common and severe. For robenacoxib, the only common side effects one might likely see isa soft stool, possibly vomiting. These symptoms resolve with discontinuation of the drug and can be mitigated by giving robenacoxib with food. 

Severe side effects are not common, but you should be aware of them because the consequences are high if they come up. The side effects of concern are the same with all NSAIDs regardless of their COX selectivity: stomach ulceration, loss of kidney function, and inappropriate bleeding. COX-preferential and COX-selective NSAIDs were developed to address these concerns; however, even these newer generations of NSAIDs should not be used frivolously. 

  • If a patient has borderline kidney function, NSAIDs should be used with caution (modified dose, monitoring blood tests, etc.) as they reduce blood flow through the kidneys. It is also important that NSAIDS not be given to dehydrated patients because of this potential side effect.

  • NSAIDs are broken down (metabolized) in the body by the liver. If the patient’s liver is not working normally due to another disease or if the patient is taking other drugs that are also removed by the liver, it is possible to “overwork” the liver and exacerbate the pre-existing liver disease. If there is any question about a patient’s liver function, another class of pain reliever should be selected. 

In any of these situations, the patient will be clearly sick with poor appetite, listlessness, possibly yellow coloration in the eyes/gums, vomiting, or diarrhea that does not resolve quickly with the discontinuation of the drug. If your pet seems ill on robenacoxib, promptly report this to your veterinarian.

Interactions with Other Drugs

Drugs of the NSAID class should not be used concurrently as the potential for the aforementioned side effects increases. For similar reasons, NSAIDS should not be used in conjunction with corticosteroid hormones such as prednisone, dexamethasone, etc.  

ACE inhibitors such as enalapril may not be as effective in the presence of NSAIDs. (ACE inhibitors are used to treat hypertension or heart failure.) This is because ACE inhibitors depend on the dilation of blood vessels in the kidneys and such dilation can be interfered with by NSAIDs.

Concurrent use of robenacoxib and the antifungal medication fluconazole may result in higher (and possibly toxic) blood levels of robenacoxib.

Concurrent use of robenacoxib with methotrexate, an agent of cancer chemotherapy, can increase the toxicity of the methotrexate significantly.

Robenacoxib should not be used in conjunction with drugs that may be potentially harmful to the kidneys. Similarly, the use of diuretics (which increase urine production), can create dehydration, and being dehydrated increases robenacoxib’s potential to damage the kidneys.

Robenacoxib is strongly bound to blood proteins and may displace other protein-bound drugs. This means that such other drugs will be in their active forms in higher amounts than expected. Toxicity could result.

Concerns and Cautions

Robenacoxib may be given with or without food. It works a little faster if given without food but if there is an upset stomach concern, giving the medication with food can prevent this side effect.

If a dose is accidentally skipped, do not double up on the next dose. Simply give the medication when it is remembered or pick it up with the next dose, allowing at least the proper interval between doses according to the label instructions.

Robenacoxib should be kept away from light and stored at room temperature. Do not refrigerate.

Do not crush or break the tablets.

Robenacoxib has not been tested in pregnant or nursing females and thus is not recommended for use in such individuals, particularly since COX-2 is important in reproductive function. This also means that pregnant women might do best by not handling this medication.

NSAIDs should not be used in animals with pre-existing liver or kidney disease.

Robenacoxib should not be used in patients with pre-existing GI ulcerations.

See more information on this product.

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