The short answer
St. John's wort may lower topiramate levels and reduce its effectiveness. The cited product and NHS guidance support avoiding the combination; no reliable dose-spacing or washout interval is established.
What this answer covers
The effectiveness record covers oral systemic topiramate; the size and onset of the interaction are not quantified.
- Supplement or preparation
- St. John's wort
- Medication ingredient
- topiramate
Evidence and practical considerations
Absorption & effectiveness
Regulatory information establishes a clinically important conditional loss-of-efficacy risk; the absence of an effect size is preserved.
- Exact scope
- St John's wort with topiramate
- Medication form and route
- oral systemic topiramate
- Timing or duration
- No onset or magnitude was quantified; enzyme induction from repeated St John's wort use is the relevant exposure.
What remains uncertain
- Exact pharmacokinetic magnitude is not reported, but exact product information identifies the interaction.
Factors that may matter: St John's wort extract composition and dose; seizure severity; concomitant antiseizure therapy.
Read the supporting source summaries
Source 1: current UK Summary of Product Characteristics
medicines.org.uk · Source check Jul 25, 2026
Population: Patients receiving topiramate; warning based on interaction potential.
The SmPC states that coadministration with St John's wort can decrease topiramate plasma concentrations and cause loss of efficacy.
The document does not provide a topiramate-specific effect size or controlled human study details.
Source 2: UK medicines safety guidance
gov.uk · Source check Jul 25, 2026
Population: People taking antiepileptic medicines.
Advises avoiding St John's wort with antiepileptics because enzyme induction may reduce plasma concentrations and efficacy.
Class-level safety communication; exact topiramate magnitude not reported.
Research assessment: · Open full citations ↗
Timing & monitoring
Concordant exact product and NHS guidance support avoidance.
- Exact scope
- St John's wort with topiramate
- Medication form and route
- Not specified in this record; applicability needs confirmation.
What remains uncertain
- No timing workaround is established.
Read the supporting source summaries
Source 1: current UK Summary of Product Characteristics
medicines.org.uk · Source check Jul 25, 2026
Population: Patients receiving topiramate.
The SmPC warns that St John's wort can lower topiramate plasma concentrations and cause loss of efficacy.
No exact effect size or washout interval is provided.
Source 2: authoritative exact-medication patient guidance
nhs.uk · Source check Jul 25, 2026
Population: People taking topiramate.
NHS says not to take St John's wort with topiramate.
Patient guidance does not quantify mechanism or magnitude.
Research assessment: Not recorded for this finding · 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: 00515e10a463e312
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.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.