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Horny Goat Weed (Epimedium spp.) is a limp excuse for Viagra, Cialis

Reuters and Bloomberg reported earlier this week on an ongoing patent battle (read: pissing match) between Pfizer and Eli Lilly & Co. relating to their erectile dysfunction drugs Viagra and Cialis, respectively.

A US Patent and Trademark Office (USPTO) appeals committee has ruled that an element of Pfizer's patent on sildenafil, the active chemical in Viagra, is invalid because the drug is insufficiently different from a traditional Chinese medicine called Yin Yang Huo or horny goat weed.

At issue is Pfizer's claim to a method for treating male erectile dysfunction. The patent appeals panel ruled that the method did not constitute a new invention because of the precedent set by Yin Yang Huo. Moreover, the board ruled that chemicals found in the herbal medicine act by the same mechanism as sildenafil by inhibition of an enzyme called phosphodiesterase-5 (PDE5). Therefore, the panel ruled, "the patent claim was the next logical step up from using the herb."

I'm not an expert in law but there are untold number of traditional remedies touted for all sorts of sexual enhancement, none of which have the convincing efficacy of the prescription PDE5 inhibitors. We may call the condition "erectile dysfunction" today and the idea of treating it may have existed for centuries but having a compound that can actually do anything about it is an invention. However, I can see the fine distinction if Pfizer claimed that the idea of treating erectile dysfunction was an invention.

You lawyers out there can weigh in but this sounds like a bunch of posturing for market share: worldwide Viagra sales were $2 billion USD last year.

But what is this horny goat weed and why is it being singled out?

Horny goat weed is the colloquial name for any number of species of plants in the Epimedium genus. Most medicinal plant accounts describe some variation on the theme that a Chinese goat herder observed an increase in mating behavior among his flock while grazing on the plant. According to North Carolina State University Department of Horticulture webpage, it grows as a deciduous perennial ground cover in USDA Zones 6, 7, and 8.

The active compound in horny goat weed is a flavonol called icariin. As shown above, icariin (left) is about as can be from sildenafil (right). Moreover, icariin is a poor inhibitor of PDE5, with a IC50 value of 5.9 μM, making it about 80-fold less potent than sildenafil (IC50 = 75 nM) (J Nat Prod 2008; 71:1513-1517.)

And remember: that's the purified compound in an isolated enzyme assay. Let's take a look at the plant extract itself, the product sold on the internet and in health food stores as horny goat weed.

This figure comes from a 2006 paper in the International Journal of Impotence Research (2006; 18:335-342) where an extract of a specifically-selected Epimedium plant was extracted and partially purified, then tested relative to sildenafil in a tissue preparation of rabbit corpus cavernosum (the crucial penile vasculature). The extract, EP-20 was tested here for its ability to increase the levels of the second-messenger molecule cGMP in response to the nitric oxide donor, sodium nitroprusside (SNP). Erectile dysfunction agents acts to inhibit the degradation of cGMP, causing the corpus cavernosum to relax and increase blood flow to the penis.

The EP-20 extract does indeed increase SNP-stimulated cGMP concentration over SNP alone, but it tops out at about 1/10 the levels stimulated by sildenafil (note the broken y-axis for scale). More importantly, the concentrations of horny goat weed extract are tremendously high: 0.1 and 0.3 mg/mL. A direct comparison with sildenafil is difficult because it is a pure compound while the EP-20 horny goat weed extract is a mixture of all the chemicals that occur in the plant.

The take-home messages?:

• The active constituent of horny goat weed, icariin, bears no structural similarity to sildenafil.


• Sildenafil is 80 times more potent than icariin in inhibiting phosphodiesterase-5


• Horny goat weed extract must be used at unbelievably high concentrations - far more than can be achieved in the bloodstream - to cause only 1/10th the effectiveness of sildenafil in a rabbit penile blood vessel model.

So, horny goat weed is far from stiff competition for a prescription PDE5 inhibitor like sildenafil. Yes, it has the potential to target an animal model of penile vasculature in a manner similar to sildenafil but it is unlikely to occur in a real-life scenario. However, there have been no head-to-head clinical trials of horny goat weed extract and sildenafil.

Going back to the Pfizer and Eli Lilly spat, there is really little comparison between horny goat weed and the active compound in Viagra.

And as for anyone thinking that horny goat weed might be a more cost-effective substitution for Viagra? You're probably better off buying a dietary supplement intentionally adulterated with other prescription drugs.