Herb-Drug Interactions: Epimedium
Epimedium brevicornu Maxim. (Berberidaceae)
Synonym(s) and related species
Barrenwort, Horny goat weed, Yin Yang Huo.
There is some taxonomic confusion within the species, and most of the commercially available material has not been properly characterised. In Chinese medicine, a mixture of species (referred to as Herba Epimedii) is often used and includes the following species (some of which may be synonyms): Epimedium koreanum Nakai, Epimedium pubescens Maxim., Epimedium sagittatum (Sieb. Et Zucc) Maxim and Epimedium wushanense T.S.Ying.
Constituents
The major constituents of all species of epimedium are prenylated flavonoids and isoflavones: the most important are icariin, epimedin A, B and C, and 6-prenylchrysin. Apigenin, luteolin, kaempferol and quercetin are also present. A multitude of other constituents, for which the pharmaceutical relevance is unclear, have been identified.
Use and indications
Epimedium is used traditionally as an antirheumatic, tonic and to enhance bone health and treat osteoporosis. The isoflavones and prenylated flavones have oestrogenic activity.
The herb is also used to enhance sexual function. Legend has it that this use was discovered after a goat herd in China found that his animals became much more sexually active after eating the herb, hence the name horny goat weed. It has therefore been widely advertised as a ‘herbal Viagra’.
Pharmacokinetics
In vitro, freeze-dried aqueous extracts of Herba Epimedii have been found to have some inhibitory effect on the cytochrome P450 isoenzyme CYP1A2, an effect thought to be related to the quercetin content of the herb. Extracts of Herba Epimedii may also inhibit (in decreasing order of potency) CYP2C19, CYP2E1, CYP2C9, CYP3A4, and CYP2D6, but the clinical relevance of this has not been established. See flavonoids, for information on the pharmacokinetics of individual flavonoids present in epimedium.
Interactions overview
Little is known. Epimedium may have additive effects with other medicines used for erectile dysfunction. For information on the interactions of the individual flavonoids present in epimedium, see flavonoids.
Epimedium + Food
No interactions found.
Epimedium + Herbal medicines
No interactions found.
Epimedium + Phosphodiesterase type-5 inhibitors
The interaction between epimedium and phosphodiesterase type-5 inhibitors is based on experimental evidence only.
Clinical evidence
No interactions found.
Experimental evidence
An in vitro study using rabbit corpus cavernosum tissue found that an aqueous extract of Epimedium brevicornum relaxed the smooth muscle of the corpus cavernosum. The extract also enhanced the relaxation caused by sildenafil, tadalafil and vardenafil.
Mechanism
Epimedium appears to have a similar mode of action to the phosphodiesterase type-5 inhibitors. In vitro, an extract of Epimedium brevicornum and one of its constituents, icariin, have been found to inhibit phosphodiesterase type-5, although both had weaker effects than sildenafil.
Importance and management
Evidence is limited to experimental studies, but what is known suggests that epimedium may potentiate the effects of the phosphodiesterase type-5 inhibitors, sildenafil, tadalafil and vardenafil. The results of in vitro studies are difficult to reliably extrapolate to humans. Nevertheless, the concurrent use of epimedium and a phosphodiesterase type-5 inhibitor could potentially lead to additive effects, which may be beneficial, but which could in theory also lead to adverse effects, such as priapism. It would therefore seem prudent to discuss concurrent use with patients, and warn them of the potential risks. Note that it is generally recommended that other agents for erectile dysfunction should be avoided in those taking sildenafil, tadalafil or vardenafil.
