Horny goat weed refers to various herb species of the Epimedium genus, and may be known as "yin yang huo." Horny goat weed contains chemicals that might help increase blood flow and improve sexual function. It also contains phytoestrogens, chemicals that act somewhat like the hormone estrogen. People use horny goat weed for erectile dysfunction (ED), sexual problems, weak and brittle bones, health problems after menopause.