What It Is:
The ripe seeds of the milk thistle plant (Silybum marianum) are used medicinally. One of the most important constituents of milk thistle is silymarin, which itself has of several chemical constituents, the most prominent of which is silybinin (silybin).
What It Does:
Diabetes
The silymarin component of milk thistle may decrease blood sugar, hemoglobin A1c, and LDL cholesterol levels when used with conventional therapy in people with type 2 diabetes. It has also been shown to reduce insulin resistance in people with coexisting diabetes and alcoholic cirrhosis.

Diseases of the Liver
Silymarin is thought to act as a liver-protectant. However, evidence of benefit in liver disease has been mixed.
One preliminary study of a specific silybinin preparation improved liver function in people with chronic active hepatitis. However, most studies in patients with hepatitis B or C have generally not shown an improvement in mortality or liver function using milk thistle or preparations of milk thistle. Similarly, in alcoholic liver disease, some preliminary clinical studies suggested that milk thistle might improve liver function and mortality. However, an analysis of several studies did not show a significant effect.

Liver protection from chemotherapy and other drugs
Preliminary evidence suggests that milk thistle extract standardized to 70% - 80% silymarin may protect the liver against damage from certain toxins, including drugs such as acetaminophen and phenytoin (Dilantin).

A recently published study (Ladas, Cancer 2010) suggests that milk thistle extract may reduce  liver toxicity due associated with chemotherapy.  Liver toxicity is often a complication of chemotherapy and limits therapy.  The study was conducted on children with acute lymphoblastic leukemia (ALL) receiving maintenance chemotherapy.  Milk thistle extract standardized to 33% silibinin was given at a daily dose of 80 mg to 320 mg of silybinin.  Milk thistle was associated with reductions in levels of liver enzymes that indicate toxicity.  Interestingly, a benefit was not seen during the short course of therapy (28 days) but one month later.
Administered intravenously (IV), silybinin may lessen liver damage due to poisoning by Amanita phalloides mushroom (death cap) — although this IV preparation is not readily available in the U.S.

Other:
Silybinin is under investigation for use in preventing or treating various forms of cancer, especially prostate cancer