Horny goat weed is an herb found in nature, which can be referred to as Epimedium. The leaves of horny goat weed are used to make medicine. Almost as many as 15 horny goat weed species are known as “yin yang huo” in Chinese medicine. Epimedium is actually made from horny goat weed extract. If you are wondering – “how long does goat weed takes to work?” Then read on to find out.