Drug Interaction Report

Fostemsavir and St John's Wort: Interaction Details

A pharmacist-reviewed analysis of the Fostemsavir and St John's Wort interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

St John's Wort

No brand names on record
+

Fostemsavir

Rukobia Rukobia®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Contraindicated
These should generally not be used together.
How we grade severity & evidence

Severity levels

  • Contraindicated: These should generally not be used together.
  • Major: Potentially serious — often needs a change or close monitoring.
  • Moderate: Can be significant — usually manageable with monitoring.
  • Minor: Usually limited clinical impact.

Evidence grades

  • Established: Well documented — supported by controlled studies or strong clinical data.
  • Probable: Good supporting evidence, though not definitively proven.
  • Suspected: Some evidence suggests this interaction, but it is not well established.
  • Possible: Limited or conflicting evidence; the interaction may occur.
  • Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 240 documented Fostemsavir interactions, 22 are rated contraindicated — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
unspecified
Evidence
probable
Severity
Contraindicated

What happens

Reduced temsavir (the active moiety of fostemsavir) exposure and possible loss of virologic response

Interaction Deep Dive

Concomitant use of fostemsavir with strong CYP3A4 inducers, such as St John's wort, is contraindicated. Significant decrease in temsavir (the active moiety of fostemsavir) exposure may occur, which may result in loss of virologic response. In a drug interaction study, coadministration of fostemsavir 1200 mg and rifAMPin (strong CYP3A inducer) 600 mg among 15 subjects resulted in decreased Cmax and AUC and by 0.24-fold and by 0.18-fold, respectively1. However, no clinically significantly drug interactions have been reported with low hyperforin (1 mg or less/day) St John's wort products and exceptions to drug interactions (including contraindications) may be considered in this case. Induction of CYP3A4 by high hyperforin (greater than 1 mg/day) products is reversible generally within 1 week after discontinuation 2.

Why it happens (mechanism)

Induction of CYP3A4-mediated metabolism of fostemsavir

Literature reports

1 report — tap to read

a) In a drug interaction study, coadministration of fostemsavir 1200 mg and rifAMPin (strong CYP3A inducer) 600 mg among 15 subjects resulted in decreased Cmax and AUC by 0.24-fold and by 0.18-fold, respectively 1.

Common questions

Can I take Fostemsavir and St John's Wort together?

Reduced temsavir (the active moiety of fostemsavir) exposure and possible loss of virologic response Always confirm with your pharmacist or prescriber before making any change.

How serious is the Fostemsavir and St John's Wort interaction?

It is rated contraindicated. These should generally not be used together.

How quickly could this interaction happen?

The documented onset is "unspecified". The timing of this interaction is not well characterized.

How strong is the evidence for this interaction?

The evidence is graded "probable". Good supporting evidence, though not definitively proven.

Questions for your pharmacist

  • Does my dose of Fostemsavir or St John's Wort need adjusting while I take them together?
  • What symptoms should prompt me to call you or my prescriber right away?
  • Does the timing of my doses matter for this combination?
  • Is there a safer alternative to one of these medications for me?

References (2)

  1. Product Information: RUKOBIA oral extended-release tablets, fostemsavir oral extended-release tablets. ViiV Healthcare (per FDA), Durham, NC, 2024. DailyMed
  2. European Medicines Agency (EMA): Assessment report on Hypericum perforatum L. herba: final – revision 1. European Medicines Agency (EMA). Amsterdam, Netherlands. 2022.
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St John's Wort

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Fostemsavir

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.