Here's a link that shows the milk thistle dosages at the bottom of the page:
http://www.acvs.org/AnimalOwners/Hea...malTopics/HMD/
Milk Thistle dosage: Dried herb: 15-20mg/pound once a day; concentrated or alcohol extract: 2-5 mg/pound every 8-12 hours.
If you get it in capsule form, you can open it and pour over the food. If you buy in liquid form, make sure it's alcohol-free and that it does not contain magnesium stearate...