Whether it really is an aphrodisiac or not is a different question.
What we do know is that the Chinese have been using preparations made from this plant for millennia and claim that it is the real deal. According to the legend, a Chinese goat herder was once puzzled as to why sometimes his animals would engage in incessant sexual activity.
Was it something they ate? "Does goat weed really work" effective is it? In present day China a water extract of horny goat weed is commonly used as a sexual tonic for both men and women and there is plenty of anecdotal evidence to suggest it works.
And what does modern science say? In the absence of controlled trials, not much.
As any plant, horny goat weed contains dozens of compounds that have potential physiological activity. There are antioxidants like quercetin, flavanols such as icariin and a host of others. Studies in mice have shown an increased testosterone level after administering plant extracts and other rodent trials have shown a decrease in levels of cortisol, a stress hormone that can depress the sex drive.
Laboratory experiments have also shown that compounds in horny goat weed interfere with an enzyme that normally degrades acetylcholine, a neurotransmitter that has been associated with sexual arousal.
Of course, in a commercial preparation of goat weed, you never really know what you are getting.