This falls under the category of herbs are wonderful and potent and can cause problems if not used correctly. The following is taken from "Herbs for Pets" by Mary
and Gregory Tilford.
"Despite much of the publicity that has been generated about this "wonder
herb" , milk thistle should not be used as a daily food supplement. Milk
thistle is a medicine that is best reserved for situations where the liver
is already under abnormal stress. When used in absence of preexisting
stress, milk thistle probably won't do any harm, but on the other hand it
might cause digest disorders or it might impair other body-cleansing
function of the liver. Many herbalists believe that it can actually slow
the metabolic functions of a healthy liver. In any case, milk thistle is
unnecessary unless there is a real and present need, and its use as a
dietary supplement constitutes waste."
In other words, if your dog is not showing signs of elevated liver enzymes on blood
work, don't use this herb as a preventative, it can actually elevate normal
(courtesy Deborah S)