Icariin is the active ingredient found in horny goat weed. It's a prenylated flavonoid compound. Flavonoids are types of antioxidants that have been shown to have many anti-aging properties. They are the same compounds found in some of the healthiest foods on Earth. Various epimedium plant species contain dozens of flavonoids (some species up to 37 different flavonoid compounds). The highest proportion is prenylflavonoids.