What is milk thistle? A plain guide

Milk thistle is a spiny, purple-flowered plant (Silybum marianum) in the daisy family, grown for its seeds. Those seeds contain a group of compounds called silymarin — mostly silibinin, also called silybin, alongside silychristin and silydianin. It's sold as capsules, tablets, tinctures and tea, most often marketed for liver support. Whether that marketing holds up against research at any given dose is a separate question, covered in full in milk thistle benefits: what the research does and doesn't show.

The plant and the part that's used

Milk thistle is a member of the Asteraceae (daisy) family — the same family as ragweed, artichokes and daisies themselves. The plant gets its name from the milky-white veins on its leaves. The part used in supplements is the fruit, commonly called the "seed," which is where silymarin concentrates. A 2018 chemistry and pharmacology review describes silymarin as a mixture of flavonolignans, with silibinin as the main and most-studied component, plus smaller amounts of silychristin, silydianin and related flavonoids.

Silymarin, standardization, and why the percentage matters more than the milligram number

Plain ground milk thistle seed contains silymarin in a modest, variable amount. That's why the research on milk thistle almost never uses raw seed powder — it uses an extract concentrated and standardized to a stated percentage of silymarin. A label that only says "milk thistle extract 1,000 mg" with no standardization percentage isn't telling you how much silymarin is actually in the capsule; a label that says "standardized to a stated % silymarin" is. How to choose a good milk thistle supplement covers what to look for on a label, and how much milk thistle should you take a day covers what doses the human research has actually used.

Forms it's sold in

Capsules and tablets of standardized extract are the most common form, and the form used in essentially all of the human research. Tinctures (liquid extracts in alcohol or glycerin) and teas made from the dried seed also exist, but silymarin's components dissolve poorly in water, which is one reason a tea infusion delivers a much less predictable amount than a standardized capsule. Some products bind silibinin to phosphatidylcholine — a "phytosome" complex, sold under names like Siliphos — specifically to get more of it absorbed; how to choose a good milk thistle supplement covers that trade-off.

Questions people ask

What does milk thistle do?

It's most commonly marketed and taken for liver support, and that's the traditional use behind most of the research on it. Whether the research actually backs that up — and at what dose — is answered directly, with sources, in milk thistle benefits: what the research does and doesn't show.

What is milk thistle used for?

Supplement labels and traditional use both point to the liver, and to a lesser extent digestion generally. That's a description of why people take it, not a claim about what it does for a given dose — see the benefits review linked above for what trials have actually measured.

What does milk thistle look like?

The live plant is tall, spiny, and has large, glossy green leaves marked with distinctive white veins, topped by purple, thistle-like flower heads. The seeds harvested for supplements are small, hard, mottled brown-and-black, and roughly teardrop-shaped.

How do you take milk thistle, and when?

That depends on the product's directions and, if you're following the research, on what the trials actually did. How much milk thistle should you take a day and when is the best time to take milk thistle cover both in detail.

Where can you buy milk thistle?

It's a common supplement, sold at pharmacies, health food and grocery stores, and online, in capsule, tablet, tincture and tea form. How to choose a good milk thistle supplement covers what to check on a label before buying any of them.

Is milk thistle healthy?

That's really two separate questions: whether it's generally well tolerated, and whether it does anything measurable. Milk thistle side effects covers the first; milk thistle benefits: what the research does and doesn't show covers the second.

Related reading: milk thistle benefits: what the research does and doesn't show, how much milk thistle should you take a day, how to choose a good milk thistle supplement, and milk thistle side effects.