Common St. John's-wort (H. perforatum) has long been used in herbalism. It was known to have medical properties in Classical Antiquity and 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 commercially for use in herbalism and medicine; other St. John's-worts 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. Standardised 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.
The red, oily extract of H. perforatum may help heal wounds. Both hypericin and hyperforin are reported to have antibiotic properties. Justifying this view with the then-current doctrine of signatures, herbalist William Coles (1626–1662) wrote in the 17th century that
"The little holes where of 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."
Hypericum perforatum may also be capable of reducing the physical signs of opiate withdrawal. Caution should be taken, as high-dosage H. perforatum interacts with a wide range of medications due to activation of the Pregnane X receptor detoxification pathway, and it also causes photosensitivity.
Hypericum extract, by inducing both the CYP3A4 and the P-glycoprotein (P-gp), can reduce the plasma concentrations of different antineoplastic agents such as imatinib, irinotecan and docetaxel, thus reducing the clinical efficacy of these drugs.
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