A thistle with a long story

Walk past a patch of milk thistle in midsummer and the leaves catch your eye first. They are glossy and spiny, with milky white veins. An old legend says the veins came from a drop of the Virgin Mary's milk. That story gave the plant names like Mary thistle and St. Mary's thistle.[1]

The purple flower heads are 1.5 to 3 inches across, on stems that often reach 3 to 6 feet. When the flowers fade, the plant makes small, shiny, brownish-black "seeds." They smell faintly of cocoa and taste oily and bitter. Botanists point out that each one is really a tiny fruit.[1] Labels call them seeds, and that is where the best-studied compounds are.

People have used milk thistle as a traditional herb since Greek and Roman times, mainly for the liver.[2] Today, European regulators list milk thistle fruit as a traditional herbal medicine. That status rests on at least 30 years of safe use, 15 of them in the EU. The traditional uses center on digestion and the liver.[3]

Scientists also study milk thistle in lab dishes, in animals and in people. This guide covers what's in the seeds and what each kind of study looks at. It also follows silybin's path through the body and explains what you may see on a label.

What's inside: silymarin and silybin

The seeds hold a group of related plant compounds. Together they are called silymarin. Silymarin makes up only about 1.5 to 3% of the dried seed,[1][2] so supplements use concentrated extracts.

Silybin (also spelled silibinin) is the biggest and best-studied part.[1] It comes in two close forms, A and B. Here is how European drug standards describe the mix in a refined extract:[2]

The seeds also contain oils, protein and other plant compounds, such as taxifolin.[1]

A standardized extract is concentrated and tested so each batch has a set amount of silymarin. Many US labels list an extract standardized to 70 or 80%.[4][5] In Europe, the standard range is 30 to 65%.[2] The numbers don't line up neatly. That's because labs can measure silymarin in more than one way. European regulators list several products where a color-based test found 140 mg of silymarin. A more exact method found about 108 mg in the same products.[2] Our guide to standardized extracts and whole herb powders explains the basics.

What lab and animal studies look at

Much of what we know about how silymarin works comes from cells in lab dishes and from animals.

Antioxidant activity comes up most often. Free radicals are unstable molecules that can harm cells. In lab tests, silymarin can neutralize them. A 2015 review also looks at Nrf2, a signal that tells cells to switch on their own defenses.[6] Related work looks at glutathione, the liver's main homemade antioxidant. Lab and animal studies measure how much of it the liver holds after silymarin.[7]

Other studies look at the outer membrane of each cell. Free radicals can damage this thin, fatty layer. In a common animal test, researchers strain the liver on purpose with alcohol or a large dose of a common pain reliever. Then they compare liver cells and their membranes in animals that got silymarin and in animals that didn't.[7][8] Other lab work looks at how liver cells build proteins.[7]

Studies like these show how silymarin behaves in cells and in animals. They also help scientists choose what to study next in people.

What human studies look at

Human research on milk thistle goes back decades. Studies in healthy volunteers have often asked practical questions. Does milk thistle change how the body handles common medicines? In a 2014 study, 9 healthy volunteers took a milk thistle extract three times a day for 14 days. Then they took four common test drugs. Milk thistle made little difference to the four major enzymes that process them.[9]

Safety has been tracked across many trials. Overall, silymarin has been well tolerated, even at 700 mg three times a day for 24 weeks. The most common complaints involved the stomach, such as nausea or loose stools.[10]

Silybin's path through the body

Silybin's trip through the body shows the liver's everyday work in action. After a dose by mouth, blood levels of silybin peak in about 2 to 4 hours.[1]

From the blood, silybin reaches the liver. Liver cells attach small tags that help it mix with water. This is the same second step of processing covered in our guide to how your body's daily detox processes work. About 80% of silybin then leaves in bile, where levels can run 60 to 100 times higher than in blood.[1]

Bile is a digestive juice your liver makes every day. It helps digest fats and some vitamins. Bile ducts carry it to the gallbladder, which stores it between meals. When you eat, the gallbladder squeezes bile into the small intestine.[11]

The trip doesn't always end there. Bile carries the tagged silybin into the gut. There, bacteria snip off some of the tags. Part of the silybin is then taken up again for another pass through the liver. A small share, about 3 to 8%, leaves unchanged in urine. About half of the silybin in the blood is cleared in roughly 6 hours.[1]

Milk thistle in teas, powders and capsules

Milk thistle is sometimes called wild artichoke.[1] The part people use is the seed, and it reaches you in a few forms.

• Tea. European regulators accept tea made from crushed seeds as a traditional way to take the herb. In Germany and Spain, the seeds are sold as tea, sometimes in herbal blends.[2] Picture a slow mug on a cool evening. Silymarin doesn't dissolve easily in water. So a cup of tea is quite different from a standardized extract.[1]
• Powder. Ground whole seed is sold as a traditional herbal product in France, Spain and the United Kingdom.[2] It carries the seed's natural silymarin level of about 1.5 to 3%.[1]
• Capsules, tablets and liquid extracts. Milk thistle supplements come in all three, and the liquid versions are made with alcohol.[12] Many use a standardized extract.
• Multi-ingredient formulas. Milk thistle also shares space with other traditional herbs in blends. You may find it in a daily formula, an evening formula or a short routine with a clear start and finish.

The list below shows what you may see on each kind of label.

Reading a milk thistle label

Milk thistle is a farm crop, and the seeds' silymarin content naturally ranges from about 1.5 to 3%.[1] That is one reason makers concentrate and standardize their extracts. In the United States, supplement makers must also follow FDA's good manufacturing practices. These rules help make sure each product has the right identity, purity, strength and makeup.[13]

• You may see the Latin name, Silybum marianum, and the plant part, usually seed or fruit. Our guide to Latin names on supplement labels explains what both tell you.
• Single-ingredient products often list the amount of extract per serving and the percent of silymarin, such as 70 or 80%.[4][5]
• In formulas with many ingredients, milk thistle often sits inside a blend with one total weight. Our guide to reading a supplement label shows how that kind of label is laid out.
• Some labels carry a seal from an outside testing group. The seal shows that the product passed that group's quality tests.[13] Our guide to third-party marks explains what each one covers.

For a full walk-through, see our guide on how to read a detox supplement label.

Whichever form you choose, it sits alongside meals with fiber and protein, plenty of water, regular movement and steady sleep. Our guide to the liver's everyday job explains the daily work those habits feed.

Frequently asked questions

Why is it called milk thistle?

The name comes from the milky white veins on its leaves. Legend links them to the Virgin Mary, which explains other names such as Mary thistle and St. Mary's thistle.[1]

Is milk thistle the same as silymarin?

Not quite. Milk thistle is the plant. Silymarin is the group of compounds taken from its seeds. Silybin is the main one. A label that says "standardized to 80% silymarin" tells you how much of the extract is silymarin.

Can I take milk thistle with my medicines?

In a small study of healthy volunteers, two weeks of milk thistle didn't meaningfully change four major enzymes that process medicines.[9] Even so, ask your pharmacist first. That goes double if you take a blood thinner, a drug for blood sugar, or a drug that needs a very exact dose.[12]

How does milk thistle fit into a daily routine?

Your liver does its work every day, fed by food, water, sleep and movement. Many people add milk thistle to that routine as a traditional herb, a habit with roots in Greek and Roman times.[2] A capsule or a cup of seed tea can take its place beside the glass of water, the evening walk and a steady bedtime.

How long can I take milk thistle?

In trials, people have taken silymarin daily for months, and it was generally well tolerated.[10] Product directions vary. Some short routines are built around a single week, so follow your label.