Milk thistle is a plant whose fruit and seeds have been used for more than 2,000 years as a treatment for disorders of the liver, bile ducts, and gallbladder. Milk thistle is native to Europe but can also be found in the United States and South America.
The medicinal ingredient found in milk thistle is silymarin, an extract of milk thistle seeds. It is an antioxidant that protects against cell damage. Silymarin contains 4 compounds: silybin (the most active), isosilybin, silychristin, and silydianin. Most research has studied silymarin or its major compound silybin, instead of the plant in its whole form.
The botanical name for milk thistle is Silybum marianum. Milk thistle is also called holy thistle, Marian thistle, Mary thistle, St. Mary thistle, Our Lady's thistle, wild artichoke, Mariendistel (German), and Chardon-Marie (French).
The ancient Greeks and Romans used milk thistle as a treatment for liver ailments and snake bites. During the Middle Ages, milk thistle was recommended to treat liver toxins. Modern homeopathic practitioners have used compounds from milk thistle seeds to treat a range of disorders including jaundice, gallstones, and peritonitis. The German Commission E, which studies the safety and efficacy of herbs for the German government, recommends milk thistle for liver damage due to toxins, cirrhosis of the liver, and as a supportive therapy for chronic inflammation of the liver.
Despite milk thistle's long history of use for liver complaints, it was not until 1968 that researchers extracted silymarin from milk thistle seeds and suggested that it might be the plant's active ingredient. Silymarin was later discovered to be a mixture of flavonolignans, a family of plant-based substances with antioxidant effects.
To research the claim that milk thistle is useful in treating cancer, its active substance silymarin and its major compound silybin have been the most widely studied ingredients.
Silymarin and silybin have been studied in the laboratory in cancer cells as well as in animal tumors of the tongue, skin, bladder, colon, and small intestine. They have been tested for their potential to:
Milk thistle is usually taken by mouth in capsules or tablets. Since it does not dissolve well in water, it is not commonly taken as an herbal tea. In Europe, the active compound silybin is given by intravenous infusion as the only effective antidote for Amanita phalloides, a rare mushroom toxin that causes deadly liver failure.
Most milk thistle supplements are measured by how much silybin they contain. Special forms of silybin made to be easily absorbed and used by the body are sold under the names Legalon, silipide, and Siliphos.
Research in a laboratory or using animals is done to find out if a drug, procedure, or treatment is likely to be useful in humans. Preclinical studies are done before clinical trials (in humans) are begun.
Silymarin, the active substance found in milk thistle seeds, has been studied in laboratory research. These studies have shown that it acts as an antioxidant by:
Silybin, the major compound found in silymarin, has been studied in laboratory experiments using cancer cell lines (cells adapted to grow in the laboratory). These studies show that silybin:
In laboratory tests using rat livers, silymarin and silybin have also been found to boost the regrowth of liver tissue.
There is 1 case report describing the use of silymarin in a patient with promyelocytic leukemia who required breaks in chemotherapy due to abnormal liver enzyme levels. During 4 months of treatment with silymarin, the patient had normal liver enzyme levels and was able to undergo chemotherapy without breaks. A second case report describes a patient with hepatocellular carcinoma whose symptoms improved when he took 450 milligrams of silymarin per day, without anticancer therapy.
A randomized clinical trial in children with acute lymphoblastic leukemia found that silymarin decreased the harmful effects of chemotherapy on the liver without working against the cancer treatment. The children taking silymarin needed fewer chemotherapy dose reductions because of side effects than the children who did not take milk thistle.
A number of clinical trials have studied milk thistle or silymarin in the treatment of patients with hepatitis, cirrhosis, or disorders of the bile ducts. These trials have used a wide range of doses with mixed results. In a trial of biologic therapy for patients with chronic hepatitis, patients taking silymarin had fewer symptoms and a better quality of life compared to patients not taking silymarin. The beneficial effects of silymarin shown in some studies suggest it might play a role in preventing hepatitis and/or liver cancer, but no clinical trials have studied the use of silymarin for prevention.
Very few bad side effects from the use of milk thistle or silymarin have been reported when taken as recommended. Several large, carefully designed studies in patients with liver disorders have found that taking silymarin may rarely have a laxative effect or cause nausea, heartburn, or stomach upset. At high doses, mild allergic reactions have been seen (more than 1,500 milligrams a day).
Silymarin has been used in pregnant women who have a bile blockage in the liver, with no toxic effects to the patient or fetus. Silymarin have also been given to children intravenously for mushroom poisoning.
It is not known if milk thistle may make anticancer medications or other drugs more effective, less effective, or have no effect when taken with them.
Milk thistle is available in the United States as a dietary supplement. Because dietary supplements are regulated as foods, not as drugs, FDA approval is not required unless specific claims about disease prevention or treatment are made.
Given the limited research done with milk thistle in patients with cancer, using it as a cancer treatment cannot be recommended except in carefully designed clinical trials.