According to WebMD, it is acceptable to give a cat baby aspirin only under the supervision of a veterinarian. Too much aspirin is toxic to cats. For a typical 8-pound cat, just one baby aspirin provides pain relief for three days.
WebMD explains that small amounts of aspirin sometimes cause appetite loss, depression and vomiting in cats. Larger amounts have more severe effects, including dehydration, uncoordinated walking, gastrointestinal bleeding, and liver and bone marrow toxicity. Aspirin should always be given with food rather than on an empty stomach. Cats should be carefully monitored for symptoms of aspiring poisoning. If any such symptoms occur, the medication should be immediately discontinued.