Milk thistle in pregnancy: evidence and safety

How does milk thistle work?

StatPearls describes silymarin acting in three ways: as an antioxidant, an anti-inflammatory, and an antifibrotic substance. On the antioxidant side it inhibits free radicals derived from the metabolism of toxic substances such as ethanol, acetaminophen, and carbon tetrachloride, scavenges free radicals directly, and stimulates protein synthesis by protecting cell membranes from free-radical damage. Its anti-inflammatory action is attributed to inhibiting the transduction cascade controlled by NF-kappa B, the complex that governs pro-inflammatory gene expression. All of that is laboratory and general-adult pharmacology, which is why StatPearls itself recommends caution during pregnancy: the human studies have not been done.

What does the evidence say about milk thistle in pregnancy?

• NCCIH states directly that little is known about whether milk thistle is safe to use during pregnancy or while breastfeeding, and that clinical trial results for liver diseases have been conflicting or too limited to allow conclusions. 1
• Silymarin is a mixture of flavanolignans, largely silybinin, silychristin, and silydianin, sold in capsules of 250 to 750 mg taken two to three times daily. LiverTox assigns it a likelihood score of E, unlikely to cause clinically apparent liver injury. 2
• Silymarin flavonolignans were undetectable in breastmilk at under 1 mcg/L in one study, and silymarin metabolites were not detected at under 0.3 mg/kg in the milk of 25 women after two weeks. Silibinin's oral bioavailability is low and variable at 23 to 47 percent. 3
• StatPearls advises that caution is recommended during pregnancy because more studies are needed on milk thistle and pregnancy, especially in humans, while noting no documented contraindications and tolerability even at 700 mg three times a day for 24 weeks in non-pregnant adults. 4
• Product quality is a distinct concern from the herb itself: NCCIH reports that some commercial milk thistle supplements contain silymarin amounts differing substantially from the label, and that some have been contaminated with pesticides and mycotoxins. 1

What's on the label?

NCCIH describes silymarin as the main constituent of milk thistle extract and a mixture of compounds rather than a single molecule, which is what a standardized percentage on a label is pointing at. The same page reports that some products have been found to contain silymarin amounts substantially different from what the label states, or to be contaminated with pesticides, microorganisms, or mycotoxins, so independent verification carries more weight here than the number on the front. StatPearls adds that flavonolignans are rapidly absorbed and eliminated after an oral standardized extract and undergo extensive phase II hepatic metabolism, which is why dosing schedules repeat through the day rather than running once.

What should I know before taking milk thistle?

Please consult your healthcare provider before starting, stopping, or stacking supplements in pregnancy, especially if you take prescription medication, live with a chronic condition, or have a procedure or delivery scheduled. Herbal supplements are not required to demonstrate purity; brands that publish third-party test results give the most confidence in what is actually in the capsule.