Sadly when cats get older, they’re often in pain. Yet ordinary painkillers don’t work on cats. Unlike humans and even dogs, cats can’t use painkillers to relieve problems associated with elderly living such as arthritis.
That means painkillers that ordinarily work on humans and dogs could have toxic effects on cats at worse, or no effect at all. Opioids in particular can even create mania in cats, although the way most cats behave, it would be hard to tell the difference.