Cats cannot tolerate aspirin as well as dogs or people. Aspirin stays in the bloodstream of cats for a long time. Almost two days after giving aspirin, a cat will still have half of it still circulating in his bloodstream. If given the dose usually given to people or dogs, aspirin will reach toxic levels in the cat quite rapidly. If aspirin is prescribed for a cat, it will generally be small doses given once every 2-3 days. Never give aspirin to a cat, unless under the direct supervision of your veterinarian, and follow the dosage instructions exactly.