Native to the Mediterranean region, Milk Thistle (Silybum marianum) is indigenous to both western and central Europe and is now found in the United States. An annual or biennial herb growing up to 10 feet tall, Milk Thistle has a large bright purple flowering head. It is often considered a pesky weed. It gains its name from the milk white veins that streak its prickly-edged leaves. The plant contains the natural chemical substance Silymarin, an active constituent highly concentrated in the fruit, but also found in the seeds and leaves. Milk Thistle has been used as both a food and a health aid for hundreds of years.