DO NOT give any cat a non-steroidal anti-inflammatory medication without consulting with your vet first. Acetaminophen is toxic to cats, aspirin is extraordinarily easy to overdose in cats because the half-life of aspirin in a cat is 72 hours (3 days!) and other non-steroidal medications do not appear to have been studied much in cats, probably due to the problems with these two common ones. For arthritis, we do sometimes use aspirin but it must be used cautiously and it is important to know the patient's medical condition well prior to its use.