The short answer
St John’s wort is not recommended with phenytoin because it can weaken treatment and put seizure control at risk. If you already use it, arrange a clinician-guided plan for changes and monitoring. Taking the products a few hours apart does not address enzyme induction.
What this answer covers
Oral Hypericum perforatum preparations with oral phenytoin; induction strength varies across products.
- Supplement or preparation
- St John’s wort
- Medication ingredient
- phenytoin
The exact medication label names this interaction
Phenytoin labeling lists St John’s wort among agents that may lower blood levels and notes that induction strength varies by preparation. Reduced concentrations can undermine seizure control; this is not simply a mineral-like binding effect in the gut.
Regulatory guidance advises against co-use
The UK regulator advises people taking antiseizure medicines not to use St John’s wort. Its highlighted seizure report involved other medicines, so it is not an exact-pair trial. The advice and the phenytoin-specific label nevertheless point in the same direction.
Spacing does not resolve the concern
The supplement can affect medication handling beyond the moment a dose is swallowed. No protective hours-apart regimen is established. Product variability is a reason for coordinated review, not evidence that an untested extract avoids the interaction.
Starting or stopping needs coordination
Tell the prescriber if you already take the herb. Phenytoin concentrations may need reassessment as interacting products change, and unsupervised medication changes can destabilize seizures. Do not independently increase phenytoin or discontinue the seizure medicine.
Evidence and practical considerations
Absorption & effectiveness
St John’s wort is not recommended with phenytoin because it can weaken treatment and put seizure control at risk. If you already use it, arrange a clinician-guided plan for changes and monitoring. Taking the products a few hours apart does not address enzyme induction.
- Exact scope
- Oral Hypericum perforatum preparations with oral phenytoin; induction strength varies across products. with Oral phenytoin; free-acid and sodium-salt formulations are explicitly distinguished.
- Medication form and route
- oral
What remains uncertain
- No universal risk rate or dose interval is established. Specific original-source limitations and access status are documented.
Factors that may matter: Exact preparation and amount; Medication formulation and drug levels; Seizure history and other treatments.
Read the supporting source summaries
Source 1: Current U.S. regulatory labeling
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: People prescribed oral phenytoin for supported seizure indications.
Labels name folic acid and St John’s wort as possible causes of lower phenytoin levels. Calcium-carbonate and magnesium-hydroxide antacids should not be taken simultaneously. Abrupt withdrawal can worsen seizures.
How this applies: Exact medication precautions with the formulation bridge explicitly retained.
A generic mineral is not every antacid salt. Free-acid and sodium formulations have different drug content; oral absorption findings do not apply to intravenous administration.
Source 2: Regulatory safety guidance
gov.uk · Source check Sep 10, 2026
Population: People with epilepsy taking antiseizure medicines.
Regulator advises against co-use after reports and consideration of possible interactions beyond known enzyme pathways.
How this applies: Supports avoidance guidance; exact phenytoin labeling separately names possible lower drug levels.
The highlighted report involved levetiracetam, lamotrigine and clobazam, not phenytoin. It does not quantify exact phenytoin risk.
Source 3: Federal supplement safety guidance
nccih.nih.gov · Source check Sep 10, 2026
Population: People considering St John’s wort while taking medication.
Lists phenytoin among medicines whose effects can be weakened.
How this applies: Reinforces the exact-medication warning and need for coordinated starting or stopping.
General guidance does not establish a product-specific degree of induction or a safe spacing interval.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Initial source-checked research draft with exact formulation limits, contrary evidence and source-access details. Clinical review not recorded. | |
| Research summary published |
Article revision: 64a8fb8f08272a22
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.