The compound icariin, found in the plant Epimedium brevicornum, commonly called horny goat weed, is a natural Phosphodiesterase-5, or PDE5, inhibitor. Icariin was the only of several natural compounds tested to inhibit PDE5 in a study published in The Journal of Natural Products.
PDE5 inhibitors treat erectile dysfunction because PDE5 inhibitors stimulate nitrous oxide/cyclic guanosine monophosphate signalling, which keeps blood flowing to the penis to maintain an erection, reports the International Journal of Impotence Research.
Other natural compounds tested for PDE5 inhibition or ED treatment include pomegranate juice, cinnamaldehyde and ferutinin, according to Reviews in Urology. Icariin was the only compound that had activity, with an IC50 (or concentration that results in half maximal inhibitorion) of 5.9 micromolar, which is about eight times less potent than that of sildenafil, the active ingredient in Viagra.
Scientists have also investigated PDE5 inhibitors as a therapy for Alzheimer's Disease through the same mechanism of action as its use for ED treatment. A study published in 2014 in the International Journal of Neuropsychopharmacology found that icariin promoted memory and learning in an AD mouse model.
Many supplements claiming to be natural herbal supplements to treat ED actually contain tadalafil, a synthetic compound found in Cialis, reports Forbes.