Sales of milk thistle in the U.S. were $108 million in 2011, up 10.2% from the prior year, according to Nutrition Business Journal, Boulder, CO. ConsumerLab.com recently tested 11 milk thistle supplements and found one product had an unacceptable amount of lead and six products did not contain expected amounts of silymarin compounds, which are believed to be the active constituents of milk thistle. While most products claimed that their milk thistle extracts were standardized to 80% silymarin, ConsumerLab.com found actual amounts to range from 48% to 67%.  

Many supplement makers appear to be using substandard milk thistle extract ingredient in their products, ConsumberLab.com said. This can happen if they rely on non-specific tests, such as UV spectrophotometric analysis, which falsely inflate the silymarin content of an extract by counting other compounds that are not silymarin. In contrast, ConsumerLab.com used a highly specific HPLC method to test the products. Some milk thistle ingredient suppliers offer a higher-priced extract (certified with the HPLC test) and a lower-priced extract (certified with the non-specific UV test). FDA does not set standards for the quality of herbal supplements nor specify how they must be tested, so manufacturers may choose either form of milk thistle.