Nest time you might want to switch to Marin. It is what Dr. Center of Cornell, one of the top liver experts, recommends. It is supposedly the most effective formulation.
I have had Lady on milk thistle for over six years with great results, but switched her to Marin recently. Entirely Pets carries it and it's very reasonable. You quarter the tablets for our little dogs, so a bottle lasts for four months.
http://www.entirelypets.com/marindog.html