Thank you for your question on ehealthforum.com
Horny goat weed leaves contain a product called icariin which is a natural Phosphodiesterase 5 enzyme inhibitor. Sildenafil, viagra act by inhibiting Phosphodiesterase 5.
PDE5 regulates blood flow normally, by its inhibiting its function; erections can be maintained for a long time. As icariin has PDE5 inhibiting action, horny goat weed is supposedly helpful for the ED. But, clinical trials need to be done before it can be recommended for usage. Thank you.