Probably a hybrid between the closely related H. attenuatum and H. maculatum (imperforate St John's wort) that originated in Siberia, the species is now found worldwide.

In modern times, its use as an antidepressant has been the focus of numerous studies and clinical trials; however, the active ingredients can be very harmful or even lethal when taken alongside other medicines.

[1] The flowers are conspicuous and showy, measuring about 1.5–2.5 cm (0.59–0.98 in) across, and are bright yellow with black dots along the edges of the petals.

[3] The most common active chemicals in Hypericum perforatum are hypericin and pseudohypericin (naphthodianthrones), and hyperforin (a phloroglucinol derivative).

[21]: 64 The genus name Hypericum is possibly derived from the Greek words hyper (above) and eikon (picture), in reference to the tradition of hanging the plant over religious icons in the home.

The herb would be hung on house and stall doors on St John's Feast Day to ward off evil spirits and to safeguard against harm and sickness to people and livestock.

[25] Many other similarly fanciful names have been used for it including devil's scourge, Lord God's wonder plant, and witch's herb.

[27] Other local names for Hypericum perforatum include balm of the warrior's wound in Somerset, penny John in Norfolk, rosin rose in Yorkshire, and touch-and-heal in Northern Ireland.

Gaterau published Description des plantes qui croissent aux environs de Montauban in 1789 which described and called the species Hypericum officinale, a name now considered to be illegitimate under the principle of priority.

It is present and native to all of the Western Asia phytogeographic region from the Aegean Islands and Turkey to Israel and Palestine in the west to Afghanistan in the east, only being absent from the Sinai Peninsula.

[31] In South America, it is found in Argentina, Chile, Uruguay, the Juan Fernández Islands, and the more temperate parts of Brazil in the southeast.

[31] The species can be found in a variety of habitats including open woods, meadows, grasslands, steppe, riverbanks, and stony or grassy hillsides and roadsides.

[21]: 88  The species thrives in areas with at least 760 mm (30 in) of rainfall per year; however, its distribution is restricted by temperatures too low for seed germination or seedling survival: an absolute minimum of −3 °C or an annual average of 9 °C.

Depending on environmental and climatic conditions, and the age of the plant, St John's wort will alter the way it grows to promote survival.

This fungus causes the stems to lose their bark (girdling) and turn brown, and dries the aboveground parts of the plant.

[38] Though Hypericum perforatum is generally avoided by mammalian herbivores, a number of insects are dependent on it and its close relatives as a food source.

Chrysolina quadrigemina and C. hyperici are two beetle species that feed on plants from the genus Hypericum, including H. perforatum.

[43][42] Zeuxidiplosis giardi, the common St. Johnswort gall midge, is a small (3 mm (0.12 in)) fly that eats H. perforatum while developing.

It replaces native plant communities and forage vegetation to the extent of making productive land nonviable or becoming an invasive species in natural habitats and ecosystems.

[48] It was thought to have medical properties in classical antiquity and was a standard component of ancient concoctions called theriacs, from the Mithridate of Aulus Cornelius Celsus' De Medicina (c. 30 CE) to the Venice treacle of d'Amsterdammer Apotheek in 1686.

[53] Some studies and research reviews have supported the efficacy of St John's wort as a treatment for depression in humans.

A 2008 Cochrane review of 29 clinical trials concluded that it was superior to placebo in patients with major depression, as effective as standard antidepressants and had fewer side-effects.

[56] A 2017 meta-analysis found that St John's wort had comparable efficacy and safety to SSRIs for mild-to-moderate depression and a lower discontinuation rate.

[48] In the United States, St John's wort is considered a dietary supplement by the FDA, and is not regulated by the same standards as a prescription drug.

[58] In China, St John's wort combined with Eleutherococcus senticosus is sold as an antidepressant under the name Shūgān Jiěyù Jiāonáng (Chinese: 舒肝解郁胶囊; lit.

[64] St John's wort can interfere (in potentially life-endangering ways) with the effects of prescription and over-the-counter drugs.

[65] Specific consequences include reduction in the effectiveness of medicines like oral contraceptives, heart medications, HIV drugs, cancer medications, and some anticoagulants;[48] breakthrough bleeding when taking oral contraceptives; and decreased effectiveness of immunosuppressants in those who have had organ transplants.

[67] However, the concentration of St John's wort's constituent chemicals (including hyperforin) can vary widely between different products,[66] and their dosage may not be properly marked on packaging.

[67] The most common side effects of St John's wort products (besides drug interactions) are stomach pain, fatigue, and restlessness.

When the pigments are extracted using ethanol, a violet-red colour is made which can be used to dye silk and wool when rinsed in vinegar.