I was wondering that question myself, almost picked some up from the store.  Looked at the ingredients - a little worried about the Vit A & D already in there, and the dosage levels.  Then, that led me to thinking that Glucosamine Chondroitin & MSM would be a better supplement.

Just my opinion.  Still thinking about it (since so many conditions have improved on the Canned food).

I have done it with a pharmaceutically pure form ... the acupuncturist say s that the typical dose is no worry of the a and d od ... the vet is more cautious and recommend s salmon oil

For some limited ingredients home-made cat food that does not have liver, cod liver oil is often helpful in providing pre-formed Vit A and D.  It i not recommended to supplement a commercially fed cat with cod liver oil.