So with the St. Valentine’s Day theme going on it makes sense to mention aphrodisiacs.  In a nutshell an aphrodisiac is anything that increases sexual desire, and draws its name from Aphrodite, the Greek goddess of love.  I most often read about aphrodisiacs as being used by men on women to increase their libido, or sexual desire.  If you expand the definition of aphrodisiac to include anything that makes sex more attainable, or likely to happen, that would open up the door to include things that allow it to happen.  You know, things that allow a dude to pitch a tent and get rid of that floppy jalopy.

That brings me to Horny Goat Weed.  It’s self explanatory.  Legend has it a shepherd was tending his flock of goats (not seagulls, you’d have to be from the ’80s to get the joke), they ate some sort of plant, then got to gettin’ busy.  So what exactly is this Horny Goat Weed?

Epimedium pinnatum by Dennis Korablyov

Horny Goat Weed is the common name for the genus Epimedium.  Other familiar names are barrenwort and rowdy lamb herb (gotta keep an eye on the lambs, too), and there are about 50 species in the genus.  The most common may be Epimedium grandiflorum, which is native to China, Korea, and Japan.  In China, where it is known as Yin Yang Huo, it is used to treat impotence as well as back pain and spasms.  There, it is the upper portions of the plant that is used; the flowers and leaves.

So, is there any truth to this?  Can Horny Goat Weed actually make goats horny?  The answer, surprisingly, is yes.  And the reason is a vowel heavy chemical called icariin.  Icariin is a secondary metabolite, meaning that is is not necessary for the plant to survive.  It is a flavonoid with a benzopyrone backbone – thats those two fused rings you see in the middle of the structure, for you non-chemists.  You hear about flavonoids all the time, as they are being touted for their anti-inflammatory and antioxidant properties.

Icariin structure

In the body icariin acts as a  phosphodiesterase type 5 (PDE5) inhibitor.  It works by blocking the breakdown of cyclic GMP in smooth muscle cells.  Since PDE5 is present in arterial walls, these inhibitors work to decrease arterial pressure, thus improving the symptoms of pulmonary hypertension (or heart failure).  One such PDE5 inhibitor that the world knows so well is sildenafil, aka Viagra, the little blue pill.  It was discovered during clinical trials that sildenafil also allows blood to flow into the corpus cavernosum penis, giving the man an erection.  Obviously there’s a larger market for helping dudes get it up than treating hypertension (not to mention the clinical trials are much cheaper and shorter), so Viagra was born.  Or erected.

A little side track here.  I think it’s worth it.  In my Pharma days I once spoke with some R&D and clinical trial managers from Pfizer, and they told how they discovered this “side-effect”.  Once a clinical trial is over, they ask the volunteers for any remaining drug back.  They were surprised as man after man said that they had tossed them, flushed them, or lost them, at a rate much higher than expected based upon other drug trials.  Hmmm, why is this?  So they asked and inquired, and eventually figured it out.  So if the men had returned the drugs like they were supposed to, Viagra as we know it might never have been born.  End of side track.

The obvious question is “how does icariin compare to Viagra?”  Not so well.  But that is to be suspected, right?  I’d like to think that Pfizer, with it’s 9 billion dollar R&D budget, to be a bit smarter than a plant.  When compared in PDE5 activity assays, icariin is nearly 100 times less potent than Viagra – 6 micromolar versus 75 nanomolar.  But, if those glycosides, those cyclic sugars attached to each side in the structure above, are replaced with much smaller hydroxyethyl groups, the potency is equal to that of Viagra (1)!

So should all the dudes out there go purchase some Horny Goat Weed?  It’s readily available, either in your local BigBoxSupermarket or pharmacy, but I wouldn’t recommend it.  That’s just me.  I’m of the opinion that diet and exercise will help in that department, and probably make you more attractive to others.  So it’s win win.  And if you do have a real medical problem, see your doctor first.

Happy Valentine’s Day!

1. Dell’Agli, Mario, et al. “Potent Inhibition of Human Phosphodiesterase-5 by Icariin Derivatives.” Journal of Natural Products 71.9 (2008): 1513-1517.