Health Benefits of Milk Thistle

Medically reviewed by Elizabeth Barnes, RDN

Milk thistle is a plant known for its anti-inflammatory and antioxidant properties. The herb has been used for hundreds of years to treat illnesses like liver disease and metabolic syndrome.

Milk thistle’s botanical name is Silybum marianum. The flowering plant is also commonly referred to as Mary thistle, holy thistle, Marian thistle, wild artichoke, Our Lady’s thistle, and St. Mary’s Thistle. Milk thistle is native to Europe and also found in South and North America.

While herbalists have touted the health benefits of milk thistle for centuries, it’s only in modern times that researchers have begun to understand its health benefits. Most existing research focuses on silymarin, the active ingredient in milk thistle believed to be the source of many of the plant's health benefits. And much research has produced mixed results.

While more research is needed to confirm the benefits of milk thistle, here are some of the ways it can potentially help your health—and what else you should know before taking it.

Dietary supplements are minimally regulated by the FDA and may or may not be suitable for you. The effects of supplements vary from person to person and depend on many variables, including type, dosage, frequency of use, and interactions with current medications. Please speak with your healthcare provider or pharmacist before starting any supplements.

May Help Treat Liver Diseases

Milk thistle is most well known for its healing and protective liver benefits. But so far, the research on how effective milk thistle is for treating liver disease is mixed. And at times, the research has been criticized for being poorly conducted.

Some research suggests milk thistle shows promise in treating nonalcoholic fatty liver disease (NAFLD). NAFLD is a condition where excess fat builds up in your liver for reasons other than heavy alcohol use. It's a condition that affects 20-30% of adults have and that is largely treated with lifestyle changes like weight loss. Untreated, NAFLD can cause liver complications and increase your risk of other conditions like cardiovascular disease and type 2 diabetes.

Milk thistle might be beneficial in the treatment of NAFLD because of its antioxidant effects. Milk thistle might also be effective against NAFLD because it reduces levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels among people with the disease. Reducing levels of the two liver enzymes (substances that speed up chemical reactions int he body) could be helpful in the treatment of NAFLD.

Milk thistle has been shown to lower liver enzyme levels in people with other liver disease, including viral infections like hepatitis B and hepatitis C, autoimmune diseases, and alcohol-associated liver disease. However, more studies are needed to determine whether the amount that liver enzymes are lowered are even clinically relevant.

Could Reduce Inflammation

Chronic inflammation is a factor in many diseases and conditions, including diabetes, heart disease, allergies, and arthritis. Milk thistle is believed the have anti-inflammatory properties.

A 2015 study found that supplementing with milk thistle may help suppress inflammation in your cells. There are also indications that milk thistle helps regulate cytokines, which are proteins responsible for the body’s inflammatory response.

Can Act as an Antioxidant

Milk thistle may have strong antioxidant qualities. Antioxidants are substances that may prevent or delay some types of cell damage. Taking in antioxidants might reduce your risk of certain diseases.

A 2020 review found that milk thistle is a multifunctional compound capable of treating a variety of different conditions, including cancer, hepatitis, NAFLD, diabetes, depression, and heart disease. Based on their findings, the researchers hypothesize that it's largely milk thistle’s antioxidant qualities that make it a good candidate for treating such a wide range of conditions.

May Help With Metabolic Syndrome

There is some evidence that milk thistle can help with metabolic syndrome, a group of conditions that increase your risk of heart disease, diabetes, and stroke.

Research has found that milk thistle extract (silymarin) can help decrease blood glucose (sugar) levels, hemoglobin A1C (A1C, the average blood sugar levels over the past three months), total cholesterol, triglycerides (a type of fat in your blood), and low-density lipoprotein (LDL) cholesterol—all of which can protect against the effects of metabolic syndrome.

May Help Manage Diabetes

Milk thistle may help people with type 2 diabetes improve glycemic control, or how well they can manage blood sugar levels. An analysis of seven studies showed that people with type 2 diabetes who took milk thistle—whether alone or with other ingredients—each day for six months had a reduction in fasting blood sugar and A1C. The effects might be even greater in the first three months of use.

Some data also indicate that milk thistle lowers insulin resistance and decreases fasting insulin levels, both of which can be beneficial in managing and protecting against diabetes.

May Aid in Treating or Preventing Cancer

Milk thistle may strengthen the efficacy of cancer drugs that treat ovarian cancer and breast cancer. Milk thistle also may help slow cancer cell growth in conditions like prostate, breast, and cervical cancer.

More research is needed to determine if milk thistle can truly help treat or prevent cancer, though. For instance, one older study found that people with prostate cancer who took a milk thistle supplement every day for 10 weeks had a longer time until their prostate-specific antigen levels increased (a sign that cancer is present). However, the milk thistle supplement contained other ingredients, such as vitamins and minerals. So it's difficult to say whether it was the milk thistle alone that was responsible for the slower cancer growth.

Currently, not much is known about how milk thistle may interact with radiation therapy and other cancer treatments.

Can Help Treat Mushroom Poisoning

Amanita phalloides, more commonly known as death cap, is a mushroom that can cause liver failure severe enough to lead to death. Death cap is one of the most poisonous mushrooms and is to blame for most of the human deaths from mushroom poisoning worldwide.

In Europe, milk thistle is given intravenously (through an IV) to treat poisoning from a death cap mushroom. Clinical trials are underway in the U.S. so the U.S. Food and Drug Administration (FDA) can determine whether the same treatment is safe and effective for use in the U.S. In the U.S., cap mushrooms are more common in California.

How To Take Milk Thistle

As a supplement, milk thistle is usually given in capsule or pill form. The seeds of the milk thistle plant contain its active ingredients, and supplements are formed using extracts from these seeds.

Some people might also eat milk thistle. Its leaves are used in salad, and the fruit of its flower can be roasted and used in place of coffee.

Dosage

There are no specific recommended dosages for milk thistle. In research settings over the past several decades, milk thistle extracts have most often been used in doses of 140 milligrams (mg), three times a day, for up to four years. Meanwhile, a 2019 review found that milk thistle was safe and well tolerated at daily doses up to 700 mg, three times a day, for up to 24 weeks. Some reported side effects at that dosage were nausea and diarrhea.

The recommended dosage has been found to vary widely between manufacturers. So on top of looking at the product's directions, you should also speak to your healthcare provider for advice about how much milk thistle to take.

Also know that the makeup of milk thistle extract can vary. The products are typically standardized based on their silymarin content, the concentration of which usually ranges between 70%-80%.

Is Milk Thistle Safe?

Milk thistle is thought to be safe, even in relatively high dosages. However, some people may experience allergic reactions to milk thistle, especially if they are allergic to other plants or flowers in the same family as milk thistle, such as daisy, ragweed, marigold, or chrysanthemum.

Milk thistle hasn’t been studied in people who are pregnant or breastfeeding, so caution is advised among these populations.

Potential Drug Interactions

There are no known medications or substances that shouldn’t be taken with milk thistle. However, there are some medications that can theoretically interact with milk thistle. This includes antidiabetes drugs since milk thistle can lower blood sugar which, on top of the diabetes medication's effects, can make blood sugar too low.

And in at least one case, the blood thinner warfarin (sold under brand names like Coumadin and Jantoven) has been shown to have an increase in its blood-thinning effect when taken with milk thistle.

Other research on potential drug interactions has had mixed results. There have been studies to show that the effects of cholesterol-lowering statins and the HIV drug Crixivan (indinavir) are affected by milk thistle, but other studies show no effect.

Other drugs—like estrogen therapy, the breast cancer treatment Nolvadex (tamoxifen), and the hepatitis C treatment Sovaldi (Sofosbuvir)—may also theoretically interact with milk thistle. However, research into these interactions has been done on animals or in test tubes.

Even though potential drug interactions are widely considered theoretical, you should always talk to your healthcare provider before taking a supplement.

What To Look For

Herbal supplements like milk thistle are not regulated or approved by the FDA. For this reason, ingredients in capsules and tablets may vary, and it’s smart to exercise caution when purchasing milk thistle.

You should consider purchasing a supplement that has been third-party tested, such as by ConsumerLab or NSF International. These organizations set strict standards for supplement quality and safety.

You'll also want to consider what form you'll want to take milk thistle. Something you may want to keep in mind: A 2013 study found that whole milk thistle and whole seeds are more likely to be contaminated with fungus than capsules, liquid extracts, or tea bags.

Can You Take Too Much Milk Thistle?

Milk thistle has been found to be safe at fairly high doses, taken multiple times a day, for extended periods of time.

However, allergic reactions are more likely to be seen at higher doses, such as 1,500 mg a day. Unpleasant side effects, such as diarrhea, are also more likely with higher doses.

Side Effects of Milk Thistle

The most common side effects of milk thistle are usually gastrointestinal and mild in nature and may include diarrhea, bloating, gas, and nausea.

Some people also experience headaches and skin reactions, like hives or a rash.

Severe adverse effects are rare. Anaphylaxis—a life-threatening allergic reaction—has been reported in some people.

A Quick Review

Milk thistle is an herbal supplement that may offer a wide variety of health benefits. It's believed that milk thistle benefits stem largely from its active ingredient, silymarin. Milk thistle has anti-inflammatory and antioxidant properties that may help with conditions such as liver disease, diabetes, and even cancer. Milk thistle is usually well tolerated and is generally considered safe. Still, because milk thistle is not regulated by the FDA—and it’s unclear whether it interacts with other substances or medications—you should connect with your healthcare provider before taking milk thistle.

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