Horny goat weed (Epimedium) is a herbal medicine that may comprise of mixtures of different related Epimedium species, which contain numerous and varied active constituents.  As herbal products are subject to variations in quality, safety and efficacy, their use should not be recommended routinely.

There is no published human information on drug interactions with horny goat weed.  There is, however, the potential for significant drug interactions due to its inhibitory action on certain subgroups of the cytochrome P450 enzyme system, although the degree to which it does so is currently unknown. Drugs with which horny goat weed may interact via this mechanism include warfarin, amitriptyline, clarithromycin and erythromycin, ciclosporin and tacrolimus, and phenytoin.  Furthermore, horny goat weed may enhance or prolong the effects of drugs that prolong the QT interval, and there is a potential for an additive interaction with phosphodiesterase‑5 inhibitors for erectile dysfunction.

Due to the lack of information of the nature and extent of interactions of horny goat weed in humans, it would be prudent to advise patients to avoid use of the product concurrently with medication until it is more widely studied.

This Q&A considers only drug interactions with horny goat weed.  It does not take into account efficacy data, which would need to be considered when treatment decisions are being undertaken.  The amount of good quality information available about herbal medicines is often limited due to the lack of regulation of these products.  Many of the safety concerns are theoretical or are based on case reports only.

Some interactions are based on the extrapolation of findings from in vitro studies on isolated enzymes.

Attachments

• UKMi_Q&A_Horny_Goat_Weed_Interactions_2020_Update · PDF · 193 KB