Many supplement makers appear to be using substandard milk thistle extract ingredient in their products. 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). The 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. Consumers normally have no way of knowing which form they purchase other than from ConsumerLab.com's independent reports.
ConsumerLab.com's Product Review of Milk Thistle Supplements can be found here. The report provides information on how to choose and use the supplements as well as test results, quality ratings, and product comparisons for the eleven supplements selected for testing.