Milk thistle is an herbal supplement used in complementary and alternative medicine. Recent research shows it may have anti-cancer capabilities.

Milk thistle is an edible plant native to Europe. It also grows in the United States and South America. Its fruit and seeds have been used for centuries to help protect the liver. It’s also been used to treat liver disorders.

The plant’s healing abilities may come from a mixture of flavonolignans called “silymarin” and its main element, which is called “silybinin.”

Research abounds on milk thistle for many health conditions. Silymarin and silibinin are well studied for liver disorders. That said, most studies on this plant’s impact on other conditions aren’t conclusive.

The Mayo Clinic says there’s “good scientific evidence” supporting the use of milk thistle for:

  • cirrhosis
  • type 2 diabetes
  • diabetic kidney disease
  • chronic liver disease

There’s “unclear scientific evidence” supporting the use of this plant for several conditions. These include:

  • allergic nasal symptoms
  • high cholesterol
  • menopause
  • fertility
  • cancer

Some laboratory studies on the herb’s cancer-fighting abilities are promising. One study on the effects of silibinin on MCF-7 breast cancer cells suggests the compound inhibits the cells’ growth and induces their death. The study concluded that silibinin might be “an effective adjuvant drug to produce a better chemo preventive response for the cancer therapy.”

A separate study on the effects of silibinin on MCF-7 human breast cancer cells also found it causes cell death and a loss of viability. The study’s results also showed that the combination of silibinin and ultraviolet light B light was more effective than ultraviolet light alone in causing cell death.

Studies have also shown that silymarin:

  • strengthens cell walls
  • stimulates enzymes that limit how toxins effect the body
  • blocks free radicals.

In addition, some of its components may improve the effectiveness of chemotherapy against breast and ovarian cancer cells. Certain components may even help prevent cancer cells from developing. The components may also slow cancer cell growth in specific cancer cell lines.

Currently, there aren’t any clinical trials being conducted for milk thistle or silymarin. A 2009 review of previous clinical trials looked at the safety and effectiveness of milk thistle. The review did find “promising results” in the plant’s ability to fight certain types of cancer. The review also determined that plant’s extracts are safe and well-tolerated. There’s a minimal risk of side effects.

Based on clinical trial data, a safe dosage is 420 milligrams per day of oral milk thistle containing 70 to 80 percent silymarin in divided doses for up to 41 months. Pregnant and breast-feeding women should avoid the herb.

Adverse side effects are usually mild. These may include:

  • nausea
  • vomiting
  • bloating
  • gas
  • changes in bowel habits
  • headaches

Symptoms of an allergic reaction are also possible. These can include hives, a rash, and difficulty breathing.

Before using this herb, it’s important to consider potential drug interactions and precautions:

  • This may interfere with the way the body metabolizes drugs that use what’s called the liver’s “cytochrome P450” enzyme system. This could increase the drug levels in the blood.
  • You should avoid this herb if you’re allergic to ragweed, marigolds, daisies, or chrysanthemums.
  • This supplement may lower blood sugar levels, and you should use it with caution when taking diabetes medications. These medications include other herbal supplements or medications that lower blood sugar.
  • Taking this herb may impact how your body absorbs other drugs.

If you take any prescription or over-the-counter medications or herbal supplements, you should check with your doctor or pharmacist before using this. Keep in mind that the U.S. Food and Drug Administration doesn’t regulate herbal supplements. As a result, potency and ingredients may vary. Only buy this supplement from a reputable brand that you trust.

Laboratory studies have shown that milk thistle may have cancer-fighting properties. Randomized, controlled human trials are needed to definitively prove its efficacy against breast cancer and other types of cancer.

At this time, there’s not enough evidence for or against the use of milk thistle to treat cancer. There isn’t evidence suggesting that it’s harmful to your health. If you’re interested in learning more about this supplement or wish to consider it as an additional therapy for breast cancer, speak with your doctor to determine if it’s a good option for you.