You might search for Marin and Denmarin on the web and read their information about use. That may find the answer for you.
I'm not on my computer right now, or I'd do it (phone doesn't cooperate well!)
Marin has Vit E in it (in addition to the milk thistle derivative)- so if that's what you're using, you should be careful about Vit E toxicity from the cat getting too much of it. What product are you using?
liquid milk thistel, nature answer. but read about taking it too long so liver does not work properly. also would like input as to whether a cat taking heart meds and thyroid meds can take milk thistle.
You might search for another vet near you, then interview them on knowledge of use of milk thistle, maybe interview then as a replacement vet while you're at it.
In my signature link Notes to Share, there is a link to Vet Interview Topics which you might find helpful if you're not happy with your vet.