St. John's Wort: How It Works, Evidence & Safety
Last updated: June 2026 · Reviewed by the FactoWiki Editorial Team
St. John's wort is a herb studied for low mood, but it is best known for serious interactions with many medications. Its presence in a formula is an important safety flag.
What is St. John's Wort?
St. John's wort is a flowering plant traditionally used for mood support. It is one of the most interaction-prone supplement ingredients.
How St. John's Wort works
Its compounds are thought to affect mood-related brain chemicals, but they also strongly activate liver enzymes that break down medications, which is the source of its many interactions.
What the evidence says
- The NIH summarises evidence for St. John's wort in depression and stresses its significant drug interactions. (NIH NCCIH)
How much do studies use?
Studied doses vary by extract. Dosage is less important here than the interaction risk, which applies at typical supplement amounts.
Safety and side effects
On its own it is often tolerated, but it can cause photosensitivity and, crucially, it changes how many drugs work.
Who should avoid St. John's Wort?
It can reduce the effectiveness of birth control, blood thinners, antidepressants, transplant and HIV medicines and more, and combining it with antidepressants can be dangerous. Talk to a doctor or pharmacist before using anything containing it.
Frequently asked questions
Why is St. John's wort a concern in supplements?
Because it interacts with a very wide range of medications, sometimes seriously. If a formula contains it and you take any medication, check with a pharmacist first.
Can I take it with antidepressants?
No - this combination can be dangerous. Do not combine them without medical guidance.