The short answer
Regular St. John's wort use may lower thyroid-hormone levels during levothyroxine treatment. Tell the prescriber when starting or stopping it so thyroid function can be monitored; the evidence does not establish blanket avoidance, automatic dose changes or a useful dose-spacing interval.
What this answer covers
Applies to systemic levothyroxine replacement, with the supporting product evidence centered on oral preparations.
- Supplement or preparation
- St. John's wort
- Medication ingredient
- levothyroxine
Evidence and practical considerations
Absorption & effectiveness
Regulatory review and updated product information support a plausible clinically relevant loss of effect, but evidence is mainly pharmacovigilance without incidence or controlled magnitude. Low-quality publication is justified by the regulatory action and narrow therapeutic index.
- Exact scope
- St. John's wort with levothyroxine
- Medication form and route
- systemic levothyroxine replacement, with regulatory evidence centered on oral products
- Timing or duration
- The signal concerns regular St. John's wort use and changes after starting or stopping; a minimum exposure duration is not established.
What remains uncertain
- No controlled exact-pair pharmacokinetic trial was located.
- Magnitude may differ substantially by Hypericum preparation and hyperforin content.
- Regulatory evidence supports monitoring but does not establish a universal dose change.
Factors that may matter: Hypericum preparation and hyperforin content; regular use; starting or stopping; baseline thyroid reserve; monitoring interval.
Read the supporting source summaries
Source 1: European regulatory pharmacovigilance assessment
ema.europa.eu · Source check Jul 24, 2026
Population: Postmarketing levothyroxine users exposed to St. John's wort products.
The regulator concluded that product information should state St. John's wort may increase hepatic clearance, lower serum thyroid-hormone concentrations, and require an increased dose.
Largely spontaneous reports and mechanistic plausibility; incidence and formulation-specific magnitude unknown; PDF fetch later returned HTTP 429 but the source was inspectable through search extraction.
Source 2: current regulator-approved product information
assets.hpra.ie · Source check Jul 24, 2026
Population: Patients using levothyroxine products.
States that St. John's wort may increase hepatic clearance and that patients may need a higher thyroid-hormone dose.
Product information summarizes the signal rather than presenting controlled trial data.
Research assessment: · Open full citations ↗
Timing & monitoring
Regulatory action supports disclosure and TSH monitoring when regular St. John's wort is started or stopped. Low-quality publication is justified by the regulatory warning, but avoidance and a universal dose change are not supported.
- Exact scope
- St. John's wort with levothyroxine
- Medication form and route
- systemic levothyroxine replacement, with product evidence centered on oral use
- Timing or duration
- Monitor after starting, stopping, or materially changing regular St. John's wort use; no dose-separation interval is supported.
What remains uncertain
- No absorption-based separation interval is supported.
- Categorical avoidance is not uniformly required by the returned regulatory sources.
- Monitoring recommendations are based on a low-certainty but regulator-recognized signal.
Factors that may matter: Hypericum preparation; hyperforin content; regularity; starting or stopping; TSH target.
Read the supporting source summaries
Source 1: European regulatory pharmacovigilance assessment
ema.europa.eu · Source check Jul 24, 2026
Population: Levothyroxine users exposed to St. John's wort.
Regulatory conclusion required product-information warnings about reduced hormone concentrations and possible increased dose requirement.
Mainly postmarketing data; no controlled monitoring schedule or universal management strategy.
Source 2: regulator-approved patient leaflet
medicines.org.uk · Source check Jul 24, 2026
Population: Patients taking oral levothyroxine solution.
Lists St. John's wort among products that can affect levothyroxine and notes monitoring and possible adjustment.
Formulation-specific leaflet; does not establish avoidance or timing.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Research summary published |
Article revision: df1adcc0ffd63a09
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
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Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.