What is silymarin and what does it do?

Silymarin is a group of plant compounds extracted from the seeds of milk thistle. It has been studied extensively in liver research. In a well designed trial in fatty liver disease it did not improve overall disease activity, but it did show a signal of reducing scarring that still needs to be confirmed.

Choose Detailed for the fuller picture, or Scientific for mechanisms and study data.

Milk thistle (Silybum marianum) has been used for liver complaints in European herbal medicine for centuries. Its seeds contain silymarin, a mixture of flavonolignans of which silibinin is the most active.

What happens in the body

In laboratory studies silymarin acts as an antioxidant, reduces inflammatory signalling and may slow the activation of the cells that lay down scar tissue in the liver.

What the evidence in people shows

In a 48-week randomised trial in adults with biopsy-proven non-alcoholic steatohepatitis, silymarin 700 mg three times a day did not meet its main goal of improving overall disease activity. However, more people taking silymarin showed less fibrosis on their biopsies and on liver stiffness scans. Because that was a secondary finding, and a later trial did not reproduce it, the honest summary is that silymarin is promising but unproven.

Safety

Silymarin is generally well tolerated. Mild digestive upset and allergic reactions, particularly in people allergic to the daisy family, have been reported.

Silymarin comprises silybin A and B, isosilybin, silychristin and silydianin. Oral bioavailability of silybin is low because of poor water solubility and extensive phase II conjugation, which is why phytosome and other formulations have been developed.

Wah Kheong et al. (2017): 99 patients with NASH and NAS of 4 or more were randomised to silymarin 700 mg three times daily or placebo for 48 weeks. The primary outcome, a 30 per cent or greater reduction in NAS, occurred in 32.7 versus 26.0 per cent (P = .467). Fibrosis improvement of one stage or more occurred in 22.4 versus 6.0 per cent (P = .023), with concordant changes on transient elastography.

The dose used (2.1 g daily) is several times higher than in most commercial products, so dose matters when interpreting the signal.