The active ingredient in avena sativa is a steroidal saponins called avenacosides that are believed to free bound testosterone, although the science of how this actually happens has not been fully explained.
Testosterone is bound by a protein called SHGB. Some studies indicate avenacosides increase active or free testosterone by freeing up the bound testosterone from SHBG so it is free to build muscle and increase sexual capacity.
A combination of avena sativa and stinging nettle was found to increase testosterone levels of 105 per cent.
For further information about avena sativa for ED treatment see:
Avena sativa as a sex enhancer
Avena sativa for erection dysfunction
Impotence Treatments – Avena Sativa
Herbs used in ED treatment