Cats are deficient in the liver enzymes needed to process aspirin. The half-life of aspirin in cats is prolonged; aspirin should be given no more frequently than every 2-3 days. When used cautiously it is a pretty good pain reliever. The first signs of toxicity are gastrointestinal ulcers and bleeding—bloody vomit or black tarry stools (indicating digested blood). Here’s what the drug handbook has to say about it:
Symptoms of acute over dosage in dogs and cats include: depression, vomiting (may be blood tinged), anorexia, hyperthermia, and increased respiratory rate. Initially, a respiratory alkalosis occurs with a compensatory hyperventilation response. A profound metabolic acidosis follows. If treatment is not provided, muscular weakness, pulmonary and cerebral edema, hypernatremia, hypokalemia, ataxia and seizures, may all develop with eventual coma and death.