Is St. John's Wort Safe with Letrozole?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Avoid combining St. John's Wort with Letrozole — St. John's Wort speeds up the liver enzyme (CYP3A4) that breaks down Letrozole, potentially reducing its effectiveness. Consult your doctor.
Why this interaction occurs
Hyperforin activates pregnane X receptor (PXR) increasing CYP3A4 expression. Result: decreased drug levels. Can reduce drug efficacy by 30-70%
Evidence
Evidence level: strong.
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 49,117 reports mentioning Letrozole. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the Letrozole + supplement combination on this page.
FAERS reports are voluntary and cannot prove that a drug caused an event. Do not use report counts to assess the safety of this specific drug-supplement pair.
Frequently asked questions
Related interactions
All St. John's Wort interactions → · All Letrozole interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.