From the Manufacturer:
Horse chestnut extract is widely used to support circulatory health. Enzymatic Therapy's Horse Chestnut Extract is standardized to contain 20% to 22% triterpene glycosides calculated as escin, just like the extracts used in the European clinical studies. Escin has been shown to decrease capillary permeability by reducing the number and size of the pores, which improves tone.
Horse Chestnut Extract also includes a number of other valuable compounds for vascular health, such as amines, coumarins, cyclitols, flavonoids, phenolic acids, sterols, and triterpenes.