Hypericum perforatum, or Saint Johnswort, contains the phototoxin hypericin. After ingestion, hypericin travels to the skin cells, where the sun's ultraviolet rays activate it. Light-colored livestock eating the plant--and humans using herbal Saint Johnswort--may suffer serious sunburn, dermatitis, skin lesions and other problems. Hypericin's effect is cumulative.
About this Author
A freelance writer, Judy Wolfe has owned Prose for the Pros, a freelance writing business, since 2006. She's been an inveterate traveler since 1961 and draws on her travel experiences to provide articles for such websites as Chincoteague Island Vacations and Berlin Dude. Wolfe holds a Bachelor of Arts in literature from California State University at Pomona.