I've been giving this to my three cats for a few months now and there's been no side effects in any of them. I started using it because my older cat takes medication for arthritis that might damage her liver, and my vet said a supplement like Denosyl can make a huge difference. After giving it to her for a month, I asked about giving it to my other cats and was given the go-ahead. I figure preventing a problem is better than treating one.