A thorough review of the literature on milk thistle current to the year 2000 can be found online in the free full text US Government Publication Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects. This review finds that the evidence to date is strongly suggestive that milk thistle helps heal the liver, although studies to date are not yet conclusive.
A more recent review of the literature can be found in Saller (2008).
Since there are no prescription drugs available to reverse liver damage--milk thistle may be your only recourse.
Lawrence V, Jacobs B, Dennehy C, et al. (2000) Milk thistle: effects on liver disease and cirrhosis and clinical adverse effects. Evidence Report/Technology Assessment No. 21. AHRQ Publication No. 01-E025. Rockville, MD.
Free Full Text
Saller R, Brignoli R, Melzer J, Meier R. (2008). An updated systematic review with meta-analysis for the clinical evidence of silymarin. Forsch Komplementmed. 15(1), 9-20.
Tamayo C, Diamond S. (2007). Review of clinical trials evaluating safety and efficacy of milk thistle (Silybum marianum [L.] Gaertn.). Integr Cancer Ther. 6(2), 146-57.