Milk thistle (Silybum marianum), also known as “Mary thistle” or “holy thistle,” is a flowering herb indigenous to the Mediterranean region as well as parts of North and South America, Europe, and Australia. In addition to its popularity as an ornamental plant, historical evidence of its use as a remedy for a variety of ailments—particularly liver problems—extends back thousands of years.1


The use of milk thistle in the United States has exploded recently, firmly establishing it among the top 10 herbal supplements sold.2 As a member of the daisy and thistle families, it is a pollen-bearing herb.3 This plant, which may grow up to six feet tall, has characteristic bright purple tuftlike flowers and bristly veined leaves filled with a milky liquid.4

Recently, attention has focused on milk thistle’s purported anti-hepatotoxic action. Chronic, all-cause liver disease results in more than 25,000 deaths a year in the United States alone, and liver failure continues to be a leading cause of death.2 Milk thistle’s effect is being studied in patients with cirrhosis, hepatitis B, and hepatitis C.

Mechanism of action

Milk thistle has a purported antioxidant effect that involves inhibiting lipid peroxidation and elevating enterohepatic levels of glutathione to help reduce free radical production, thereby reducing or preventing oxidative hepatocyte damage.2 The active ingredient of milk thistle is silymarin, a composition of three isomers (silybin, silychristin, and silydianin). Milk thistle seeds contain an average of 1.5%-3.0% of this group of compounds.4Silymarin is also credited with increasing hepatocyte synthesis, which would facilitate regeneration of damaged liver.5

Frequently referred to as liver-cleansing agents, silymarin seeds and extract are thought to alter the outer liver-cell membrane structure by binding to receptor sites so that toxins cannot enter.3 Silymarin also potentially stimulates RNA polymerase A, which enhances ribosome protein synthesis and leads to activation of the regenerative capacity of the liver.3

Early North American folk medicine considered a steeped tea brew of milk thistle to be a tonic used principally for “strengthening the digestive organs” and especially helpful in cases of dyspepsia and loss of appetite.6

Scientific data

Results are mixed. A meta-analysis of 13 placebo-controlled trials involving 915 patients found that overall, there was no evidence “supporting or refuting” milk thistle as a liver disease therapy. Concluding that the herb “could potentially affect liver diseses,” the researchers called for additional research. 7

Previous studies, however, have shown benefit. A five-month trial with similar patients showed normalization of liver function tests (LFTs) (alanine aminotransferase, aspartate aminotransferase, and bilirubin) and histologic improvement on biopsy.3 Another double-blind study with varying doses of active silymarin showed similarly improved LFTs and reduced disease-related complications, hastened recovery, and shortened hospital stays in patients with acute viral hepatitis.3

The NIH’s National Center for Complementary and Alternative Medicine continues to study this and other herbal therapies in randomized controlled trials to further define the evidence-based utility of this product.

Dosage, economics, and how supplied

The active component of silymarin seeds is poorly soluble in water, reaching only about 10% solution after overnight soaking.4 When shopping for milk thistle, bear in mind that studies have shown increased bioavailability when the silymarin compound is bound to a carrier molecule of phosphatidylcholine.3 Milk thistle is supplied in capsules, tablets, and teas. Standard capsule doses are 420-800 mg orally per day and may be divided into two to three doses.3

According to the German Federal Institute for Drugs and Medical Devices, this dose should contain 200-400 mg of pure silymarin component.4 Like many herbal products, cost varies widely depending on the type of vendor. Typical costs for a month’s supply of milk thistle are $10-$20.

One study used a combination of a-lipoic acid, silymarin, and selenium to treat patients awaiting transplantation for severe liver disease. The success of this treatment allowed clinicians to defer transplantation at a cost savings of more than $350,000 per patient.3

Safety and side effects

Though generally well-tolerated, milk thistle has been known to have a mild laxative effect and to worsen menstrual cramping.3 Pregnant women, women planning to become pregnant, lactating women, and children should not use milk thistle, since safety in these populations is not well-established.

As mentioned, this is a pollen-bearing plant, and individuals with allergic histories to members of the ragweed, daisy, marigold, or chrysanthemum families should not use this herb.5


Milk thistle appears to be relatively safe, cost-effective, and potentially beneficial as adjunct therapy for liver toxicities due to viral or other toxic disease. Data are unclear as to its absolute efficacy for improving liver function and hepatocyte regeneration. Of course, milk thistle should not be used to obviate the need for traditional medical therapies. To date, there are no data to substantiate monotherapy with milk thistle for liver disease treatment.

However, given the significant human and financial burden caused by liver disease in this country, a relatively safe and inexpensive therapy to augment traditional medicine should be seriously considered in appropriate cases.


1. National Center for Complementary and Alternative Medicine. Milk thistle. Available at Accessed May 9, 2007.

2. Holistic Healing and Alternative Health Care Solutions. Herbal health newsletter. Available at Accessed May 9, 2007.

3. Fetrow CW, Avila JR. Professional’s Handbook of Complementary & Alternative Medicines. Springhouse, Pa.: Springhouse Corp; 1999:430-433.

4. Petit JL. Alternative medicine. Clinician Reviews. 2000:10:72-74.

5. Skidmore-Roth L. Mosby’s Handbook of Herbs & Natural Supplements. 3rd ed. St. Louis, Mo.: Elsevier Mosby; 2006:717-720.

6. Gunn JC. Gunn’s Newer Family Physician. Philadelphia, Pa.: Wm. H. Moore & Co; 1873:811.

7. Medical News Today. Milk thistle extract treatment for liver disease is found to be ineffective. Available at Accessed May 9, 2007.

Ms. Sego is a staff clinician at the Veterans Administration Hospital in Kansas City, Mo., where she practices adult medicine and women’s health. She also teaches at the nursing schools of the University of Missouri and the University of Kansas.