Hypericum
Hypericum is a genus of about 400 species of flowering plants in the family Clusiaceae, subfamily Hypericoideae (formerly often considered a full family Hypericaceae). The genus has a nearly world-wide distribution, missing only from tropical lowlands, deserts and polar regions. All members of the genus may be referred to as St. John's-worts, though they are also commonly just called hypericums, and some are known as tutsans. The marsh St. John's-worts are nowadays separated in Triadenum.
St. John's-worts vary from annual or perennial herbaceous herbs 5-10 cm tall to shrubs and small trees up to 12 m tall. The leaves are opposite, simple oval, 1-8 cm long, either deciduous or evergreen. The flowers vary from pale to dark yellow, and from 0.5-6 cm in diameter, with five (rarely four) petals. The fruit is usually a dry capsule which splits to release the numerous small seeds; in some species it is fleshy and berry-like.

# Uses of Hypericum
Some species are used as ornamental plants and have large, showy flowers. Numerous hybrids and cultivars have been developed for use in horticulture, such as Hypericum  moserianum (H. calycinum  H. patulum) and Hypericum calycinum cv. 'Hidcote'.
St. John's-worts can occur as nuisance weeds in farmland and gardens. On pastures, some can be more than a nuisance, causing debilitating photosensitivity and sometimes abortion in livestock. The beetles Chrysolina quadrigemina, Chrysolina hyperici and Agrilus hyperici like to feed on Common St. John's-wort (H. perforatum) and have been used for biocontrol where the plant has become an invasive weed.
Hypericum species are the only known food plants of the caterpillar of the Treble-bar, a species of moth. Other Lepidoptera species whose larvae sometimes feed on Hypericum include Common Emerald, The Engrailed (recorded on Imperforate St. John's-wort, H. maculatum), Grey Pug and Setaceous Hebrew Character.

## Medical properties
Common St. John's-wort (H. perforatum) is since long used in herbalism. It was already known to have medical properties in the Classical Antiquity. It was a standard component of theriacs, from the  Mithridate of Aulus Cornelius Celsus' De Medicina (ca. 30 CE) to the Venice treacle of d'Amsterdammer Apotheek in 1686. Folk usages included oily extract ("St. John's oil") and Hypericum snaps.
H. perforatum is the most potent species and it is today grown and collected commercially for use in herbalism and medicine; other St. John's-worts probably also possess interesting properties and chemical compounds but are not well researched. As these secondary compounds appear to be related to deterring herbivores, they are present in varying and unpredictable quantities. Still, a number of high-yield cultivars have been developed.
Two main compounds of interest have been studied in more detail: hyperforin and hypericin. However, the pharmacology of H. perforatum is not resolved, and at least its antidepressant properties are caused by a wide range of factors interacting. As psychiatric medication, it is usually taken as pills, or as tea. Few standardized preparations are available, and research has mainly studied alcoholic extracts and isolated compounds. What research data exists supports a noticeable effect in many cases of light and medium depression, but no significant improvement of severe depression and OCD.
Another common use of H. perforatum is as oily extract. The ruby-red oil appears to be strongly antibiotic, assisting healing of wounds, first-degree burns and concussions. Both hypericin and hyperforin are considered to be antibiotic by modern science. But, justifying it with the then-current doctrine of signatures, herbalist William Coles wrote in the 17th century already that
"The little holes whereof the leaves of Saint Johns wort are full, doe resemble all the pores of the skin and therefore it is profitable for all hurts and wounds that can happen thereunto."
As mentioned above, there is evidence that St. John's-worts can act as abortifacients; it interferes with the Combined oral contraceptive pill. Complications have also occurred in human patients. High-dosage H. perforatum interacts with a wide range of medications due to activating the Pregnane X receptor detoxification pathway, as well as causing photosensitivity. It is strongly recommended not to take St. John's-wort during pregnancy or when tanning, and it has caused a few deaths in patients undergoing anti-HIV/AIDS and cancer therapy. Extremely high doses (rarely reached with OTC preparations) are hepatotoxic.

# Selected species