Hypericum has a nearly worldwide distribution, missing only from tropical lowlands, deserts and polar regions. All members of the genus may be referred to as St. John's wort, though they are also commonly just called hypericum, and some are known as tutsan.

Among the many medicinal herbs used throughout the long history of Occidental culture, St. John’s wort, Hypericum perforatum L., has always been and still is of great interest. From the time of the ancient Greeks down through the Middle Ages, the plant was considered to be imbued with magical powers and was used to ward off evil and protect against disease. As a practical folk-remedy, it has been used widely to heal wounds, remedy kidney troubles, and alleviate nervous disorders, even insanity, and recent research makes a provocative statement about the ancient uses of St. John’s wort by showing that it is a modern protector against depression and virus infection.

In the western United States, St. John’s wort is especially prevalent in northern California and southern Oregon, hence one of its common names, “Klamath Weed”. Because of the known photosensitizing properties of the plant, which can be toxic to cows and sheep.

St. John’s wort was used in early pre-Christian religious practices in England, and it has many legends written about it. Because of its bright yellow color, it was often associated with the sun and was often used for purposes of divination–for every situation from longevity to test one’s chances for matrimony. To predict their chances for marital bliss, young girls were in the habit of plucking a sprig of flowers–if the flowers were fresh in the morning, their chances were good, if wilted, a dismal outcome was predicted.

The tops of Hypericum were also considered effective for keeping away undesirable influences and bringing luck. For instance, one belief was that bringing the flowers of St. John’s wort into the house on a midsummer eve would protect one from the evil eye, banish witches, promote good fortune and protect the house from fire.