Oil of Primrose

[OldWays]

Two years ago, I dropped a capsule of Oil of Primrose and my longhaired black cat ate it.  Since then, he knows when I get my vitamines and he sits waiting for his.  I began giving him one capsule 500 g of coldpressed oil of primrose.  He loves the stuff, and his coat is full, thick and shiny.
However, last fall I had to rush him into emergency for the urinary feline disease.  They catheterized him and the vet said we had to keep him on Veteranian OS food, which we have.  I give him tuna, packed in water, and some treats. His kidneys have been better since, however, a month ago, I had to rush him back for popping his knee cap.  Which the vet said will happen again, and we need to help him not get up on high surfaces by giving him a little stool of such. I have a very good Vet, and he said, this happens , especially with the breed of kitty he is. I've asked young vet's in school if the primrose could have made a negative impact.  I guess that's what I'm asking again.  Anyone know?
Thank you in Advance, and
Brightest of Blessings

[OldWays]

If I can ask,,,is there something in that vitamin that is helpful to want kitties to eat it.  I purchased some vitamins for him, but, was not successful in having him ingest it.  I tried fish, his primrose, even putting it in his treats, and he refuses to take it.  
The can of tuna is share between 3 cats, so he's only getting a partial portion, however, almost everyday.  I seen a program on W5 or so, in regards to feeding pets with the generic pet food, and since then I try everything I can to suppliment their food.
Thankyou for your time and I will check out your website.  As you can tell, we pamper our pets, as they bring us such joy, love and laughter.
Blessings,
Willow