My Lady has been on seizure drugs for seven years and they are notoriously hard on the liver. I started giving her milk thistle within the first six months and all these years later, her liver values are still in the normal range. My vet is amazed.
I used to use milk thistle from GNC. A member on another forum I belong to has a dog with MVD and was was told by Dr. Center from Cornell to buy Marin. She said the formula is much more effective. For those of you who are not familiar with her, Dr. Center is considered to be the expert on liver disease.
http://www.vet.cornell.edu/faculty/Center/
You can get Marin at Entirely Pets. It's really reasonable. Lady only gets 1/4 tablet once a day so a bottle lasts for four months.
http://www.entirelypets.com/marindog.html