Milk Thistle vs TUDCA: Why We Chose the Botanical

Milk thistle is a flowering plant, and the part people take is an extract of its seeds. The main group of active compounds is called silymarin, a mixture rather than a single molecule (NCCIH). It has a long history of traditional use for the liver, and it has been studied in people for decades.

TUDCA, short for tauroursodeoxycholic acid, is a bile acid. Your body makes small amounts of it, and it is closely related to ursodiol (also called UDCA), a prescription drug the FDA has approved for dissolving certain gallstones and for a liver condition called primary biliary cholangitis (ursodiol capsule label; ursodiol tablet label). TUDCA itself is sold in the U.S. as a supplement, not as an approved drug.

Long story short: one is a plant extract, the other is a bile acid. They get compared because both come up in liver conversations, but they work in very different ways.

How each one works

Silymarin is mostly studied for its antioxidant activity, meaning it may help cells handle the everyday wear and tear of oxidative stress. That is the reason it keeps showing up in liver research, since the liver processes so much of what we eat, drink and take.

TUDCA works through bile. Bile is the digestive fluid your liver makes to break down fats, and bile acids like TUDCA and its cousin ursodiol change how that fluid behaves. That is also why ursodiol is prescribed for specific gallstone and bile-duct conditions, and why anyone with a gallbladder issue should get a doctor's input before trying a bile acid on their own (ursodiol label).

• Milk thistle: a botanical extract, silymarin is the active mix
• TUDCA: a bile acid, a close relative of the prescription drug ursodiol
• Both are sold as supplements, and neither is FDA-approved to treat any condition in supplement form
• Most of the human research on TUDCA comes from people with specific liver or bile conditions

NAC is here because it comes up in the same searches. It works differently again.

Sources: NCCIH, the ursodiol label, and MedlinePlus on NAC as an acetaminophen antidote.

We scored each one from 0 to 10 on four criteria, and the total is the average: human research record (how long and how much it has been studied in people), tolerability (how well people handle it in studies), relevance to healthy adults (how much of the research is in people like you rather than in disease populations), and fit for a daily botanical supplement. None of these scores says an ingredient works; they say how much we actually know. Milk thistle: research 8, tolerability 8, healthy-adult relevance 6, daily fit 9, for 7.8. NAC: research 8, tolerability 8, healthy-adult relevance 6, daily fit 5.5, for 6.9, because most of its long record is as a hospital medicine rather than a daily botanical. TUDCA: research 5, tolerability 7, healthy-adult relevance 3, daily fit 7.5, for 5.6.

Milk thistle scores highest because it has the longest research record and a good tolerability profile, even though the results on the liver itself are mixed. A 2025 Cochrane review found little to no difference in serious side effects with silymarin, and called much of the evidence on its other effects uncertain (Cochrane, 2025). That is an honest picture, and it is still more than TUDCA has for healthy people.

The TUDCA research that exists is real, it is just narrow. In a 24-week trial in people with primary biliary cholangitis, TUDCA and ursodiol performed similarly on a key liver lab marker (Ma et al., 2016), and a small trial tested it in cirrhosis (Pan et al., 2013). A four-week study in adults with obesity found changes in liver and muscle insulin sensitivity (Kars et al., 2010). And a combination medicine containing TUDCA (taurursodiol) was withdrawn from the ALS market after a larger trial failed to confirm its benefit (review, 2024). None of that tells you what TUDCA does in a healthy person taking it daily.

"Milk thistle, when taken orally, appears to be well tolerated."

National Center for Complementary and Integrative Health

Here is something most comparison posts skip: the bottle matters. NCCIH notes that milk thistle supplements sold in the U.S. and elsewhere have been found with silymarin amounts very different from what the label states, and with quality problems in some products (NCCIH). The same basic caution applies to any supplement, TUDCA included.

So whichever one you are considering, a few habits help you get what you are paying for.

• Look for a clear amount per serving on the Supplement Facts panel, not just a proprietary blend total
• Choose brands that tell you where the product is made and who makes it
• Be wary of anything promising to detox, cleanse or repair your liver, since no supplement can make that claim
• Keep the bottle and bring it to your appointment so your doctor can see exactly what you take

We print every ingredient in The Plug Pills on the label, including the milk thistle amount, so you and your doctor can see exactly what is in each serving. We think that kind of transparency is the least you should expect from anyone, us included.

TUDCA and NAC often end up in the same shopping cart, but they do different jobs. TUDCA is a bile acid, while NAC is a building block your body uses to make glutathione, one of its main antioxidants. NAC is also an approved medicine in hospitals, where it is used as the antidote for acetaminophen overdose (MedlinePlus). Neither one replaces the other, and neither replaces the basics.

This is also why we chose milk thistle for The Plug Pills, alongside B vitamins and our 13-herb Plug Blend. Our choice was about the length of the human track record and how well it is tolerated, not a promise of results, and at 141 mg per serving the Pills are a daily support product rather than a high-dose milk thistle supplement. You can read how the Pills and the Drink differ in our Drink vs Pills breakdown.

Before you add either one

1. Talk to your doctor first if you have gallbladder disease, gallstones or any liver condition, since TUDCA acts on bile.
2. Bring a list of your medications and supplements, including anything that affects bile or your liver.
3. Skip both if you are pregnant or breastfeeding unless a clinician has cleared them.
4. Stop and call your doctor if you notice yellowing skin or eyes, dark urine or new belly pain.