Epimedium, also known as horny goat weed. This is an example of Epimedium grandiflorum, a different species to the one used in the study (Image: Sphl, Wikimedia Commons)

The soft green heart-shaped leaf of the horny goat weed could hold the key to a new drug for treating erectile dysfunction. Researchers say the Viagra alternative could be as effective as the famous blue pill, but have fewer side-effects.

Mario Dell’Agli of the University of Milan, Italy, and colleagues tested four plants which are used as natural aphrodisiacs in traditional cultures to establish their potential as alternatives to Viagra.

Viagra’s active compound, sildenafil, works by inhibiting an enzyme called phosphodiesterase-5 (PDE5). Because PDE5 helps control blood flow to the penis, inhibiting PDE5 promotes male erection.


Dell’Agli and his colleagues tested the four plants in vitro to see how efficient they were at inhibiting PDE5. Just one – Epimedium brevicornum, also known as horny goat weed and Bishop’s Hat – had an effect. This confirmed previous studies showing that icariin, a compound found inside the horny goat weed, is a PDE5 inhibitor.

The fifth compound

Sildenafil, however, is 80 times more effective at inhibiting PDE5 than icariin. Dell’Agli and his team extracted icariin from the plants, and produced six modified versions of it, which they also tested on PDE5. The most efficient of these, compound 5, “works as well as Viagra”, says Dell’Agli.

A drug made from compound 5 could also cause fewer side effects than Viagra.

In addition to PDE5, sildenafil affects other phosphodiesterases, including some that are essential to sight and heart function. As a result, people who have heart problems are not advised to take Viagra and patients who do take the drug sometimes suffer disturbances to their eyesight.

Preliminary tests suggest that compound 5 does not affect other phosphodiesterases, meaning it may not have the same side effects as Viagra.

Eat your weeds

Compound 5 will now have to go through lengthy clinical trials before it can be approved as a drug. It could be 10 years before it reaches the market.

In the meantime, “if people eat horny goat weed, I think it can be beneficial because it contains icariin,” says Dell’Agli. “But it will not be as effective as Viagra.”

Horny goat weed is found in the wild in China, Asia and Europe.

The research was supported by private funds, but Dell’Agli declined to provide details.

Journal reference: Journal of Natural Products, DOI: 10.1021/np800049y