From the Manufacturer:
Vitex contains several different constituents, including flavonoids, iridoid glycosides, and terpenoids. The whole fruit extract, rather than one of its individual constituents, appears to be necessary for the medicinal activity of vitex. Vitex does not contain hormones. The benefits of vitex stem from its actions upon the pituitary gland—specifically on the production of a hormone called luteinizing hormone (LH). This indirectly increases progesterone production and helps regulate the menstrual cycle. Vitex also keeps prolactin secretion in check. The ability to decrease mildly elevated prolactin levels may benefit some infertile women as well as some women with breast tenderness associated with premenstrual syndrome (PMS).