Thistle, particularly milk thistle (Silybum marianum), is widely used to support the hepatic (liver) system. The primary active constituent, silymarin, has been the subject of numerous scientific studies. Silymarin is a complex of flavonolignans known for antioxidant, anti-inflammatory, and hepatoprotective properties. Several clinical trials and meta-analyses suggest that milk thistle extract may help improve liver function markers in people with liver diseases such as alcoholic liver disease, non-alcoholic fatty liver disease (NAFLD), and hepatitis. For example, some studies have reported decreases in liver enzyme levels (e.g., ALT, AST) and improvements in symptoms among patients taking milk thistle extracts compared to controls.

However, while there is a substantial body of evidence supporting the use of milk thistle for liver health, results are not entirely consistent across all studies. Some trials have failed to demonstrate significant clinical benefits, and methodological limitations are present in several investigations. Nonetheless, the overall evidence base is stronger than for most herbal remedies, and milk thistle is one of the most scientifically validated botanicals for liver support. Traditional use of other thistle species for liver health is less documented and less studied, so the evidence rating applies primarily to milk thistle. Milk thistle is generally considered safe with a low incidence of side effects.