Found this, still wondering now:
"Milk thistle provides hepatocellular protection by stabilizing hepatic cell membranes. It alters the structure of the outer cell membrane of the hepatocytes in such a way as to prevent the penetration of the liver toxins into the interior of the cell. The stimulation effect on nucleolar polymerase A results in an increase in ribosomal protein synthesis, and thus increases the regenerative ability of the liver and the formation of new hepatocytes. Other actions include interruption of enterohepatic recirculation of toxins and regeneration of damaged hepatocytes (2). An animal study performed in rats demonstrated a reduction in kidney damage following administration of cisplatin without diminished anti-tumor activity (7). Other studies indicate the flavonoids in milk thistle have anticancer effects by inducing G1 and S phase arrest in cells (8). Anecdotal data suggests that milk thistle may prevent liver damage from hepatotoxic medications including butyrophenones, phenothiazines, and phenytoin (6)."
www dot mskcc.org/mskcc/html/69303.cfm
www dot en.wikipedia.org/wiki/Enterohepatic_circulation