Milk thistle extract has almost no known side effects and may be used by most people, including pregnant and breast-feeding women. In fact, it has been recommended as a treatment for itching due to poor gallbladder function during pregnancy.¹⁷ Since silymarin stimulates liver and gallbladder activity, it may have a mild, transient laxative effect in some people. This will usually cease within two to three days.

There is one case report of a 57-year-old Australian woman experiencing several episodes of nausea, abdominal pain, vomiting and weakness after taking a milk thistle preparation.¹⁸ This case is so atypical, however, that the Adverse Drug Reactions Advisory Committee of Australia questioned whether the product taken might not have contained other herbs or additives that could be responsible for the adverse reaction. In addition, rare but serious allergic reactions to milk thistle have been reported.