St. John's Wort

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Information about St. John's Wort

St. John’s wort is a popular herbal medication and extract derived from the plant Hypericum perforatum which is purported to be beneficial in depression and anxiety. 

Common Names: St. John’s wort, hypericum, Klamath weed, goatweed

Latin Names: Hypericum perforatum

Background and history

  • St. John’s wort is a plant with yellow flowers that has been used in traditional European medicine as far back as the ancient Greeks. The name St. John’s wort apparently refers to John the Baptist, as the plant blooms around the time of the feast of St. John the Baptist in late June.
  • Historically, St. John’s wort has been used for a variety of conditions, including kidney and lung ailments, insomnia, and depression, and to aid wound healing.
  • Currently, St. John’s wort is most often used as a dietary supplement for depression. People also use it as a dietary supplement for other conditions, including menopausal symptoms, attention-deficit hyperactivity disorder (ADHD), and obsessive-compulsive disorder. It is used topically for wound healing.
  • The flowering tops of St. John’s wort are used to prepare teas, tablets, capsules, and liquid extracts. Topical preparations are also available.

Liver safety of St. John's Wort

St. John’s wort has not been implicated convincingly in cases of clinically apparent, acute liver injury, although it may increase the hepatotoxicity of other agents by herb-drug interactions that alter drug metabolism.  

Mechanism of action of St. John's Wort

St. John’s wort is an extract prepared from the flowers and leaves of a flowering plant native to Europe commonly known as St. John’s wort, Tipton’s weed or chase-devil (Hypericum perforatum).  St. John’s wort is a yellow flowering perennial herb indigenous to Europe that has been introduced elsewhere.  Its name refers to the Saint’s day on which it is typically harvested – June 24th. 

Western St. John's-Wort (Hypericum formosum) on Kaien Island, British Columbia, Canada
Western St. John's-Wort (Hypericum formosum) on Kaien Island, British Columbia, Canada

Clinical use of St. John's Wort

The genus name Hypericum means “above the picture” in reference to its use to ward off evil by hanging the plant over a religious picture or icon.  St. John’s wort is an invasive weed in many countries where it has been introduced, where it can be toxic to livestock.  St. John’s wort has been used widely as an herbal treatment for depression.  It is available as an over-the-counter herbal in the United States, but in other countries is actively prescribed for mild-to-moderate depression. 

Active ingredients

Extracts of St. John’s wort contain many polyphenols, including flavonoids (rutin, hyperoside, isoquercetin, quercitrin, quercetin and others), phenolic acids, naphthodianthrones (hypericin, pseudohypericin, protohypericin and others), and phloroglucinols (hyperforin, adhyperforin).  The active principle responsible for the antidepressant effects of St. John’s wort is not known, the most likely candidates being hypericin, pseudohypericin and hyperforin. 


In controlled trials, St. John’s wort has shown evidence of an antidepressant effect in patients with mild to moderate depression. 

Side effects of St. John's Wort

Side effects can occur with St. John’s wort including gastrointestinal upset, dizziness, confusion, fatigue, anxiety and photosensitivity.  Importantly, St. John’s wort has effects on the cytochrome P450 system (induction of CYP 3A4 and 2C9) as well as the major drug transport protein – P-glycoprotein.  As a result, St. John’s wort has major drug interactions, particularly with birth control pills, transplant rejection agents, antiretroviral agents, digoxin, platelet inhibitors, anticoagulants and psychotropic agents.  Caution should be taken and specific interactions sought when using St. John’s wort with other medications.

How Much Do We Know?

  • There has been extensive research on St. John’s wort, especially on its use for depression and on its interactions with medications. It has been clearly shown that St. John’s wort can interact in dangerous, sometimes life-threatening ways with a variety of medicines.

What Have We Learned?

  • The results of studies on the effectiveness of St. John’s wort for depression are mixed. For more information, see the NCCIH fact sheet St. John’s Wort and Depression.
  • St. John’s wort has also been studied for conditions other than depression. For some, such as ADHD, irritable bowel syndrome, and quitting smoking, current evidence indicates that St. John’s wort is not helpful. For others, such as menopausal symptoms, premenstrual syndrome, and obsessive-compulsive disorder, the evidence is inconclusive.

Drug interactions and effects

  • St. John’s wort can weaken the effects of many medicines, including crucially important medicines such as
    • Antidepressants
    • Birth control pills
    • Cyclosporine, which prevents the body from rejecting transplanted organs
    • Digoxin, a heart medication
    • Some HIV drugs including indinavir
    • Some cancer medications including irinotecan
    • Warfarin, an anticoagulant (blood thinner).
  • Taking St. John’s wort with certain antidepressants or other drugs that affect serotonin, a substance produced by nerve cells, may lead to increased serotonin-related side effects, which may be potentially serious.
  • St. John’s wort may cause increased sensitivity to sunlight. Other side effects can include anxiety, dry mouth, dizziness, gastrointestinal symptoms, fatigue, headache, or sexual dysfunction.

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