Emtricitabine and Horsetail: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Emtricitabine
Horsetail
No brand names on record
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.
What happens
Reduced emtricitabine efficacy; increased HIV RNA viral loads
Interaction Deep Dive
Avoid coadministration of emtricitabine and horsetail, as reduced emtricitabine exposure may occur and result in increased HIV RNA viral loads. A 75-year-old man that was on a fixed-dose combination of emtricitabine, tenofovir disoproxil fumarate, and efavirenz had 2 consecutive detectable viral loads (71 copies/mL and 1590 copies/mL) after initiating horsetail1.
Why it happens (mechanism)
Unknown
Literature reports
1 report — tap to read
a) In a similar situation, a 75-year-old man that was on a fixed-dose combination of emtricitabine, tenofovir disoproxil fumarate, and efavirenz had 2 consecutive detectable viral loads (71 copies/mL and 1590 copies/mL) 4 months and 9 months after switching to this antiretroviral regimen. Upon questioning, the patient had also been taking horsetail. A viral load test performed 1 month after discontinuation of horsetail showed the viral load resuppressed at less than 50 copies/mL with all subsequent viral loads remaining undetectable. The Drug Interaction Probability Scale rated this drug interaction as possible 1.
Common questions
Can I take Emtricitabine and Horsetail together?
Reduced emtricitabine efficacy; increased HIV RNA viral loads Always confirm with your pharmacist or prescriber before making any change.
How serious is the Emtricitabine and Horsetail interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
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.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Emtricitabine or Horsetail 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 (1)
- Cordova E, Morganti L, & Rodriguez C: Possible drug-herb interaction between herbal supplement containing horsetail ( Equisetum arvense) and antiretroviral drugs. J Int Assoc Provid AIDS Care 2017; 16(1):11-13.
Keep reading about Emtricitabine
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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