Silymarin is believed to work in two main ways. It provides antioxidant activity, which may help protect liver cells. It also helps reduce oxidative stress, a type of cell damage, inside the liver.
The Liver Therapies Patient Guide also lists possible support for liver cell regeneration, anti-inflammatory effects, and improvement in certain liver enzyme levels. These effects are described as possible, not proven.
The research is mixed. According to the National Institutes of Health, results from clinical trials of milk thistle for liver disease have been mixed, and two carefully designed studies found no benefit. A 2012 trial found that two higher-than-usual doses of silymarin were no better than placebo in people with chronic hepatitis C who had not responded to standard antivirals. Some studies suggest milk thistle may protect the liver from toxins, but some of those studies were poorly planned.
For these reasons, Legalon is a complement to, not a substitute for, standard liver disease care.