Aspirin (acetylsalicylic acid) in Pets

Aspirin causes severe stomach ulcers and kidney injury in dogs, and is exceptionally toxic to cats. Learn feline sensitivities, toxic doses, and warning signs.

What It Is

Aspirin (acetylsalicylic acid, or ASA) is a classic human non-steroidal anti-inflammatory drug (NSAID) and salicylate derivative used to relieve pain, reduce fever, and inhibit platelet aggregation. Aspirin works by non-selectively inhibiting cyclooxygenase (COX-1 and COX-2) enzymes, which decreases the production of protective prostaglandins. While prostaglandins drive inflammation, they also maintain gastric mucosal blood flow, stimulate protective stomach mucus production, and regulate renal perfusion. Inhibiting them leaves the digestive tract vulnerable to deep ulceration and impairs kidney function.

Where Pets Come Across It

Most pet poisonings result from well-meaning owners administering human headache or arthritis tablets to relieve their pet's limping, or dogs chewing through plastic medicine bottles and blister packs left on bedside tables. Aspirin is also present in combination cold medications, buffered powders, effervescent tablets, and topical skin formulations such as salicylic acid wart removers and oil of wintergreen (methyl salicylate).

Which Animals Are Affected

Dogs and cats are both affected, but cats are much more vulnerable. Merck explains that cats are deficient in the liver enzyme glucuronyl transferase and excrete aspirin very slowly, with a half-life of about 37.5 hours, so repeated doses build up.

Published Toxic Dose

Merck describes ulcers in dogs given repeated aspirin: doses of 25 mg/kg every 8 hours caused stomach erosions in half of treated dogs within two days. A single large ingestion of about 450 mg/kg can cause vomiting, high body temperature, panting, seizures or coma. Cats are far more sensitive than dogs, and Merck does not give a safe dose for them; any aspirin eaten by a cat should be discussed with a vet.

Signs and How Soon They Appear

Signs usually begin within 1 to 4 hours of ingestion with gastrointestinal distress, including vomiting, anorexia, drooling, and abdominal pain. As ulcers form, vomitus may contain dark altered blood resembling coffee grounds, and stools become black and tarry (melena). High doses trigger rapid breathing (tachypnea) from direct stimulation of the respiratory centre, followed by severe metabolic acidosis, high body temperature, weakness, muscle tremors, acute kidney injury, seizures, and coma.

What to Do Right Now

Seek veterinary care immediately. Bring the medicine packet or bottle so the veterinary team can calculate the exact milligram strength of acetylsalicylic acid ingested. Do not attempt home emesis without explicit veterinary direction. Treatment typically involves clinic decontamination, intravenous fluids to maintain renal perfusion, gastroprotectant medications (such as omeprazole and sucralfate), and urinary alkalinisation in severe overdoses.

Salicylate sensitivity in cats and hepatic glucuronidation

The extreme sensitivity of domestic cats to aspirin and other salicylates is a classic example of feline unique pharmacology. Domestic felines have very low levels of UDP-glucuronosyltransferase enzymes in the liver. In most mammals, this enzyme pathway conjugates drugs like aspirin into harmless water-soluble metabolites that are rapidly excreted through the kidneys. In cats, this pathway is severely deficient, meaning the drug circulates in the bloodstream for nearly two full days from a single dose. Consequently, what seems like a trivial children's dose to a human can cause fatal organ damage in a feline companion.

Frequently Asked Questions

Sources and References

• Merck Veterinary Manual: Toxicoses from Human Analgesics in Animals
• Pet Poison Helpline: Aspirin

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