Horny goat weed is an herb. The leaves are used to make medicine. As many as 15 horny goat weed species are known as "yin yang huo" in Chinese medicine.
Horny goat weed is commonly used by mouth for sexual performance problems, such as erectile dysfunction (ED) and low sexual desire. It is also used for weak back and knees, joint pain, arthritis, mental and physical fatigue, and memory loss along with many other conditions. But there is limited scientific research to support any of these uses.
Horny goat weed contains chemicals which might help increase blood flow and improve sexual function. It also contains phytoestrogens, chemicals that act somewhat like the female hormone estrogen. This might reduce bone loss in postmenopausal women.