Common St. Johnswort is a vigorous competitor in pastures, rangelands, and natural areas, displacing native and/or more desirable forage species throughout the West. At its peak densities in the mid-1900s, it was considered the leading cause of economic loss to California agriculture, attributed both to stock fatalities and the loss of pasture and rangeland. Glands on the weed’s foliage produce hypericin, an oil and phototoxin. Upon ingestion, animals become sensitive to sunlight, often leading to dermatitis, inflammation of the mucus membranes, itching, swelling, blisters, and open sores. Animals with lighter pigmentation, a thinner fur or wool covering, and softer skin (young) are affected most, as are nursing animals whose mothers are exposed to hypericin. If consumed in large quantities, starvation, dehydration, and death may occur. Humans may also experience the same reactions when using herbal remedies containing hypericin.