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Onsior or Robenacoxib is a nonsteroidal anti-inflammatory drug [NSAID] that effectively relieves acute pain and post soft-tissue surgery and inflammation in cats. It travels rapidly through the bloodstream to selectively target the tissues involved in inflammation and pain. The drug takes effect and exits the blood quickly, to reduce potential side effects. Causes fewer gastrointestinal side effects by selective inhibition of prostaglandin formation.

Key Benefits

  • Single dose quickly alleviates acute pain for 24 hours
  • Relieves pain associated with orthopedic and soft tissue surgery
  • Tasty flavored tablets
  • Available in a 3-pack blister packet

How it works

Onsior contains the active ingredient robenacoxib, an NSAID (non-steroidal anti-inflammatory drug) that targets the COX-2 enzyme that causes pain. It spares the COX-1 enzyme to protect your pet's gastrointestinal tract and blood platelet functions. The tablet is quickly absorbed from the gastrointestinal tract and is effective for 24 hours.


Onsior® (robenacoxib) injection is indicated for the control of postoperative pain and inflammation associated with orthopedic surgery, ovariohysterectomy and castration in cats ≥ 4 months of age; for up to a maximum of 3 days.

Onsior® (robenacoxib) Tablets for Cats are indicated for the control of postoperative pain and inflammation associated with orthopedic surgery, ovariohysterectomy and castration in cats ≥ 5.5 lbs (2.5 kg) and ≥ 4 months of age; for up to a maximum of 3 days.


View Onsior Drug Facts Sheet.

Always provide "Information for Cat Owners" Sheet with prescription. Carefully consider the potential benefits and risks of Onsior tablets and other treatment options before deciding to use Onsior tablets. Use the lowest effective dose for the shortest duration consistent with individual response.

The dose of Onsior tablets is 0.45 mg/lb (1 mg/kg) orally once daily, for a maximum of 3 days. See dosing chart for dosage directions.

Dosage Directions:

For oral use in cats ≥ 5.5 lbs and ≥ 4 months of age; for up to a maximum of 3 days. Tablets are not scored and should not be broken.

Body weight 6 mg Onsior (robenacoxib) Tablet
5.5 to 13.2 lbs (2.5 to 6 kg) 1 whole tablet once daily
13.3 to 26.4 lbs (6.1 to 12 kg) 2 whole tablets once daily

Do not use in cats weighing less than 5.5 lbs, as cats less than 5.5 lbs cannot be accurately dosed.

The first dose should be administered approximately 30 minutes prior to surgery, at the same time as the pre-anesthetic agents are given. Onsior tablets may be given with or without food.

In cats ≥ 5.5 lbs and ≥ 4 months of age, subsequent doses can be given via the oral tablet, or interchanged with subcutaneous injection for a maximum of 3 total Onsior doses over 3 days, not to exceed one dose per day.

Note the dose of Onsior tablets and Onsior injection are different.

If a second and third tablet dose is dispensed to the client to administer at home, doses should be dispensed in the dispensing envelope with the attached Information for Owner Sheet intact. Do not remove Information for Cat Owner Sheet. Record when the first dose was administered on the dispensing envelope. Cats weighing ≥ 13.3 lbs may require two blister cards, each dispensed in an individual dispensing envelope.


Onsior tablets should not be used in cats that have a hypersensitivity to robenacoxib or known intolerance to NSAIDs.


Not for use in humans. Keep this and all medications out of reach of children. Consult a physician in case of accidental ingestion by humans. For oral use in cats only.

All cats should undergo a thorough history and physical examination before the initiation of NSAID therapy. Appropriate laboratory tests should be conducted to establish hematological and serum biochemical baseline data prior to administration of an NSAID. Owners should be advised to observe for signs of potential drug toxicity (see Adverse Reactions and Animal Safety) and be given an "Information for Cat Owners" Sheet about Onsior tablets.

Do not administer Onsior tablets or injection in conjunction with any other oral or injectable NSAID or corticosteroid.


When using NSAIDS such as Onsior, the use of fluid therapy during surgery is recommended to decrease potential renal complications (see Adverse Reactions, Post-Approval Experience).

Appetite should be monitored in cats receiving Onsior.

Stop administration of Onsior if appetite decreases or if the cat becomes lethargic.

The use of Onsior tablets has not been evaluated in cats younger than 4 months of age and weighing less than 5.5 lbs, cats used for breeding, or in pregnant or lactating cats. Cats receiving Onsior should weigh at least 5.5 lbs.

The use of Onsior in cats with cardiac disease has not been studied. Onsior has been shown to prolong the QT interval.

As a class, cyclo-oxygenase inhibitory NSAIDs may be associated with gastrointestinal, renal, and hepatic toxicity. Sensitivity to drug-associated adverse events varies with the individual patient. Cats that have experienced adverse reactions from one NSAID may experience adverse reactions from another NSAID. Patients at greatest risk for adverse events are those that are dehydrated, on concomitant diuretic therapy, or those with existing renal, cardiovascular, and/or hepatic dysfunction. Anesthetic drugs may affect renal perfusion; approach concomitant use of anesthetics and NSAIDs cautiously. Appropriate monitoring procedures (including ECG, blood pressure, and temperature regulation) should be employed during all surgical procedures. The use of parenteral fluids during surgery is recommended to decrease potential renal complications when using NSAIDs perioperatively.

If additional pain medication is needed after a daily dose of Onsior, a non-NSAID/non-corticosteroid class of analgesic may be necessary. Concurrent administration of potentially nephrotoxic drugs should be carefully approached and monitored. NSAIDs may inhibit prostaglandins which maintain normal homeostatic function. Such anti-prostaglandin effects may result in clinically significant disease in patients with underlying or pre-existing disease that has not been previously diagnosed. NSAIDs possess the potential to produce gastrointestinal ulcerations and/or gastrointestinal perforations. Do not use Onsior concomitantly with other anti-inflammatory drugs, such as NSAIDs or corticosteroids. Consider appropriate washout times when switching from one NSAID to another or when switching from corticosteroid use to NSAID use. Onsior tablets and Onsior injection are safe to use interchangeably when given once a day for a maximum of 3 days in cats ≥ 4 months of age and ≥ 5.5 lbs. Note the dose of Onsior tablets and Onsior injection are different.

The use of concomitantly protein-bound drugs with Onsior has not been studied in cats. Commonly used protein-bound drugs include cardiac, anticonvulsant and behavioral medications. The influence of concomitant drugs that may inhibit metabolism of Onsior has not been evaluated. Drug compatibility should be monitored in patients requiring adjunctive therapy. Concurrent medications used during the field study with Onsior tablets included antiparasiticides, anesthetics, pre-anesthetic medications, and antibiotics.

The effect of cyclo-oxygenase inhibition and the potential for thromboembolic occurrence or a hypercoagulable state has not been evaluated. It is unknown whether cats with a history of hypersensitivity to ß lactam drugs will exhibit hypersensitivity to Onsior. Robenacoxib is poorly soluble in water and in acid solutions readily degrades to form -lactam. In cats, lactam is a metabolite of robenacoxib. Additionally, lactam is a degradation product that increases over the shelf-life of the tablets. Neurologic signs have been associated with the use of ß lactam drugs; it is unknown if the lactam in robenacoxib may cause similar neurologic signs (see Animal Safety). Occurrences of seizures, ataxia, and nystagmus have been associated with the use of Onsior.

Robenacoxib may prolong the QT interval; the associated risk of developing ventricular arrhythmia is unknown. The use of robenacoxib with other drugs shown to prolong the QT interval is not recommended. Commonly used drugs that prolong the QT interval include antihistamines, prokinetic drugs, and certain anesthetic drugs. As a class, it is unknown if the use of NSAIDs in asthmatic cats can cause acute exacerbations, as seen in some humans.

Adverse Reactions:

In a controlled field study, a total of 249 male and female cats representing 6 breeds, 6 months to 13 years old, weighing 5.5 - 15 lbs were included in the field safety analysis. The following table shows the number of cats exhibiting each observation.

Adverse Reactions in the Postoperative Pain Field Study1
Adverse Reaction Onsior 6 mgTablets for Cats (robenacoxib) n=167 Placebo (vehicle control) n=82
Incision site bleeding 7 1
Incision site infection 6 2
Inappetance, weight loss 4 2
Decreased activity, lethargy 4 1
Vomiting 4 1
Cystitis, hematuria 3 0
Hematachezia, diarrhea 3 1
Hair loss, excoriation, bruising 2 0
Respiratory, cardiac arrest 1 0
Incoordination, weakness 1 1
Death 0 1

1 Cats may have experienced more than one of these signs during the study.

The most commonly reported adverse reactions were surgical site bleeding, infected surgery sites, lethargy, vomiting and inappetance. Changes in the clinical pathology values were not considered clinically significant.

Information for Cat Owners:

Onsior, like other drugs of its class, is not free from adverse reactions. Owners should be advised of the potential for adverse reactions and be informed of the clinical signs associated with drug intolerance. Adverse reactions may include vomiting, diarrhea, decreased appetite, dark or tarry stools, increased water consumption, increased urination, anemia, yellowing of gums, skin or white of the eye due to jaundice, lethargy, incoordination, seizure, or behavioral changes. Serious adverse reactions associated with this drug class can occur without warning and may result in death (see Warnings and Adverse Reactions). Owners should be advised to discontinue Onsior tablets and contact their veterinarian immediately if signs of intolerance are observed. The vast majority of patients with drug related adverse reactions have recovered when the signs are recognized, the drug is withdrawn, and veterinary care, if appropriate, is initiated.


Effectiveness was demonstrated using Onsior tablets in a masked, placebo-controlled, multi-site field study involving client-owned cats. In this study, 249 cats presenting for ovariohysterectomy or castration in conjunction with an onychectomy (forelimbs only) were randomly administered Onsior tablets, or a placebo. Drug was administered approximately 30 minutes prior to surgery along with pre-anesthetic medications and continued once daily for two additional treatments. Effectiveness was evaluated in 244 cats and field safety was evaluated in 249 cats. A statistically significant difference in the proportion of treatment successes in the Onsior tablets treatment group (137/164 or 83.5%) compared to the placebo, vehicle control group (43/80 or 53.8%) was observed. Twenty-seven out of 164 robenacoxib cases (16.5%) and 37 out of 80 placebo cases (46.2%) were treatment failures. Statistically significant differences for pain elicited on palpation at the spay or castration incision site, behavior following social interaction and posture score at various post-surgical time points were also observed. The results of the field study demonstrate that Onsior tablets, when administered for a maximum of three days, are effective and well-tolerated for the control of postoperative pain associated with onychectomy, ovariohysterectomy and castration in cats.


Store at controlled room temperature, between 59° and 77°F (15-25°C).

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