Horny goat weed may cause an allergic reaction in people who are allergic to the Epimedium species, states Drugs.com. It may also cause slower blood clotting, possibly lower blood pressure and may make hormone sensitive cancers worse, according to WebMD.
Horny goat weed may not be safe when used for long periods of time or in large amounts, states WebMD. This herb is often found in products for osteoporosis, erectile dysfunction and a variety of other health conditions. As of 2015, scientific research does not support the use of this herb for medical purposes. Recommended and safe dosage amounts of horny goat weed are not available.