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Quick guide to Lady's Thistle

Scientific Name: Milk Thistle

Other Names: Cardui mariae, Carduus marianum, Holy Thistle, Lady's Thistle, Legalon, Marian Thistle, Mariendistel, Mary Thistle, Our Lady's Thistle, Silimarina, Silybin, Silybum marianum, Silymarin, St. Mary Thistle

Who is this for?

Milk thistle contains several chemicals with possible medical effects. Most current research focuses on one of them, silymarin, which may have specific protective effects on cells in the liver. In multiple human, animal, and laboratory studies, it has shown differing degrees of effectiveness for protecting the liver from damage caused by alcohol, chemicals, drugs, diseases, and poisonous plants. Silymarin and other chemicals in milk thistle are believed to protect liver cells in several different ways:

Silymarin has antioxidant properties. Antioxidants are thought to prevent or lessen damage to body cells that is caused by a chemical process called oxidation.

Anti-inflammatory effects of silymarin help keep liver cells from swelling in response to injury.

Silymarin seems to encourage the liver to grow new cells, while discouraging the formation of inactive fibrous tissue.

By changing the outside layer of liver cells, silymarin may also keep certain harmful chemicals from getting into liver cells.

Milk thistle may also cause the immune system to be more active.

Silymarin and other chemicals from milk thistle have also been tested in laboratory studies involving various types of human cancer cells. In general, they seem to interrupt cancer cell division as well as shortening the time that cancer cells live. They may also stop or limit the formation of new blood vessels that supply tumors. Most research has centered on breast cancer and prostate cancer, but milk thistle may also be useful in treating other cancers such as leukemia. The application of another milk thistle chemical, silibinin, to the skin of laboratory animals has protected the animals against the development of skin cancer — either before or immediately after exposure to damaging radiation. In addition, some chemicals from milk thistle may increase the effectiveness of current anticancer drugs. Some of these anticancer effects are being studied in early-phase human trials, but none is confirmed, yet.

Milk thistle and chemicals derived from it are being studied for a number of additional potential effects. For example, in animal studies and one small study in humans, milk thistle produced modest reductions in cholesterol levels. Results of separate laboratory studies show that milk thistle may help to protect the heart muscle from damage caused by certain drugs. However, these potential uses have not been well-studied in humans nor have they been proved in animal or laboratories studies.

When should I be careful taking it?

Women with hormone-dependent conditions such as endometriosis, uterine fibroids, and cancers of the breast, ovaries, or uterus should not take or use milk thistle plant extract due to its possible estrogenic effects.

Pregnant women should not take milk thistle because of its possible estrogen-like effects, which could interfere with normal fetal development.


Men who have prostate cancer should not take milk thistle without the approval of a doctor.

Some cases of skin rash from touching milk thistle plants have been reported in the medical literature. Milk thistle belongs to the same family of plants as chrysanthemums, daisies, and ragweed. Individuals who are sensitive to those plants may also be sensitive to milk thistle.

What side effects should I watch for?

No severe side effects have been reported from taking milk thistle. Doses greater than 2500 mg (2.5 grams) per day have been reported to have a slight laxative effect, however.

Occasionally, individuals taking milk thistle have reported:

  • Abdominal bloating
  • Diarrhea
  • Gas
  • Loss of appetite
  • Nausea
  • Stomach upset

What interactions should I watch for?

Milk thistle may interfere with the effectiveness of estrogen by blocking an enzyme that breaks down estrogen in the body. As a result, estrogen products may not be as effective in reducing the symptoms of menopause or preventing osteoporosis. In addition, oral contraceptives may not be as effective and an unintended pregnancy could result.

Because it is broken down by certain enzymes in the liver, milk thistle may possibly interfere with the effectiveness of prescription drugs that are processed by the same enzymes. Some of these drugs are:

  • Allergy drugs such as Allegra
  • Antianxiety drugs such as alprazolam (Xanax), diazepam (Valium), and lorazepam(Ativan)
  • Antifungal drugs such as ketoconazole (Nizoral) and Sporanox
  • Antiviral drugs such as Crixivan, Fortovase, and Norvir
  • Cancer drugs such as etoposide (VePesid), paclitaxel (Onxol, Taxol), vinblastine, or vincristine
  • Drugs for high cholesterol such as lovastatin

No interactions have been reported between milk thistle and non-prescription drugs, other herbal supplements, or foods. However, because few reliable studies of milk thistle have been conducted, its possible interactions with drugs, foods, and other dietary supplements are not completely understood. Be sure that your doctor or pharmacist is aware of all the prescription and non-prescription medicines you take before you begin to use milk thistle or any other herbal supplement.

Some interactions between herbal products and medications can be more severe than others. The best way for you to avoid harmful interactions is to tell your doctor and/or pharmacist what medications you are currently taking, including any over-the-counter products, vitamins, and herbals.

Should I take it?

Milk thistle gets its name from the thick white fluid that seeps from the leaves when they are broken. Its long leaves have prominent white veins and sharp spines that can scratch unprotected skin. Purple or pink flowers that grow individually on tall stems bloom in mid-to-late summer. Each milk thistle plant has up to 50 flowers; each flower contains about a hundred seeds. The seeds resemble dandelion seeds, since they are attached to feathery structures that blow in the wind. Thought to have originated in areas around the Mediterranean Sea and possibly regions of India, milk thistle is now found growing wild in most parts of the world with moderate temperatures — including Canada, Europe, and the United States. It grows as an annual in cooler climates or a biennial in bush that can be as tall as ten feet and that has a very strong taproot. Because it spreads rapidly, grows in marginal areas such as vacant lots, and crowds out other plants; milk thistle is often considered to be a weed. It may poison cattle and other livestock that eat large amounts of whole plants.

Currently, milk thistle seeds are the part most commonly used in medicine. All the parts that grow above the ground may be used, however, to make extracts. In the past, milk thistle products have been used to stimulate the flow of breast milk in women who were breast-feeding infants. It was also a folk remedy for depression. Its leaves, roots, and stems have been eaten as a vegetable in some parts of the world, and its seeds may be toasted and boiled into a coffee-type beverage.

Dosage and Administration

Most of the milk thistle products used in human studies were standardized to contain between 70% and 80% of silymarin. Standardization by the manufacturer should assure the same amount of active ingredient in every batch of the commercial preparation. Standardization of herbal products is not required by the U.S. Food and Drug Administration (FDA), so not every product sold in the United States will contain the same amounts of active ingredients.

Standardized commercial preparations of milk thistle are common in Europe. An injectable form of silymarin is also used by European physicians, but it is not available in the United States. In North America, milk thistle is sold most often as an extract, which is a concentrated liquid preparation made by soaking one or more of the chopped or mashed “aerial” parts of milk thistle plants (flowers, leaves, seeds, and stems) in a liquid such as alcohol. This semi-liquid preparation is then strained to filter out the solid parts. It can be packaged as a liquid or dried and made into capsules or tablets. Extracts made only from the seeds of milk thistle are thought not to have estrogen-like effects. The active chemicals in milk thistle do not dissolve very well in water, so making a tea from dried milk thistle is not very useful.

A typical daily dose of milk thistle extract is 200 mg to 420 mg in one, two, or three doses. In clinical studies, the most common length of dosing was 4 weeks to 8 weeks.


Milk thistle is effective at protecting the liver from toxic substances. In many parts of the world, it is used as an emergency measure to prevent death from eating poisonous mushrooms and it is taken to prevent or lessen liver damage from some medications and industrial chemicals. It may also have some usefulness for liver diseases such as hepatitis and cirrhosis — including alcohol-induced liver damage.Also used to promote milk production for women who are breast-feeding, milk thistle has mild estrogenic effects that may make it useful for treating prostate cancer. It is thought to reduce cholesterol to some degree, but more studies are needed to prove its usefulness in the treatment of high cholesterol.


Due to the estrogen-like effect that may be associated with taking milk thistle plant extract, pregnant women and women with hormone-dependent conditions should not take it. Men who have prostate cancer should not take it without the approval of a doctor.

Touching milk thistle plants may cause a skin rash.

Side Effects

Side effects from using milk thistle are reported to be mild. They include general gastrointestinal upset.


Milk thistle may interfere with the effects of estrogen replacement therapy and oral contraceptives due to its possible estrogenic effects. The effectiveness of drugs that are broken down by the same enzymes that break down milk thistle may also be affected. If you take prescription medications, ask your doctor or pharmacist about possible interactions before you begin to take milk thistle.


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(Note: The above information is not intended to replace the advice of your physician, pharmacist, or other healthcare professional. It is not meant to indicate that the use of the product is safe, appropriate, or effective for you.)

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