Many species of Epimedium have aphrodisiac qualities associated with content of icariin. According to legend, this property was discovered by a Chinese goatherder who noticed sexual activity in his flock after they ate the weed. It is sold as a health supplement, usually in raw herb, tablet, or capsule form and sometimes blended with other supplements. The over-exploitation of wild populations of epimedium for use in traditional Chinese medicine is having potentially serious consequences for the long-term survival of several species, none of which is widely cultivated for medicinal purposes.
The "active ingredient" in epimedium is icariin, which can be found in standardized extracts from 5% up to 60% potent. Strengths above that are usually reserved for laboratory use.
Icariin is purported to work by increasing levels of nitric oxide, which relax smooth muscle. It has been demonstrated to relax rabbit penile tissue by nitric oxide and PDE-5 activity. Other research has demonstrated that injections of Epimedium extract directly into the penis of the rat results in an increase in penile blood pressure.
Like sildenafil (the erectile dysfunction drug commonly sold as Viagra), icariin, the active compound in Epimedium, inhibits the activity of PDE-5. In vitro assays have demonstrated that icariin weakly inhibits PDE-5 with an IC50 of around 1 μM, while sildenafil has an IC50 of about 6.6 nM (.0066 μM) and vardenafil (Levitra) has an IC50 of about 0.7 nM (.0007 μM). Measured differently, the EC50 of icariin is approximately 4.62 μM, while sildenafil's is .42 μM. With the weak potency of Epimedium, and its unknown oral bioavailability, the amount of Epidemium extract necessary to have any effect is unclear from the literature.
A recently published Italian study modified icariin structurally and investigated a number of derivatives. Inhibitory concentrations for PDE-5 close to sildenafil could be reached. Moreover, the most potent PDE-5 inhibitor of this series was also found to be a less potent inhibitor of phosphodiesterase-6 (PDE-6) and cyclic adenosine monophosphate-phosphodiesterase (cAMP-PDE), thus showing it to have more specificity for PDE-5 than sildenafil.
Epimedium has been shown to up-regulate genes associated with nitric oxide production and changes in adenosine/guanine monophosphate balance in ways that other PDE5 inhibitors do not. Epimedium may have potential to help sexual dysfunction and osteoporosis.
Epimedium was used in a patent infringement case to rescind parts of the U.S. Viagra patent based on historic use in Chinese medicine. The specific claim was that Viagra was the first medical example of a PDE5 inhibitor which treated erectile dysfunction and therefore claimed patent protection from all similar PDE5 inhibitors. Patent examiners used Epimedium as an example of prior use and rescinded those portions of the patent, however, the Viagra patent still protects the manufacturing process and/or chemical formula for sildenafil.