Drug & supplement interaction

Fennel + Estradiol

Estradiol brand names: Estrace, Gynodiol, Vagifem, Estring, Vivelle, and others.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

Estradiol falls into 2 drug categories that Fennel can interact with; the most serious is rated moderate. Theoretically, fennel might increase levels of drugs metabolized by CYP3A4.

Read the full interaction report
SeverityModerate
LikelihoodPossible
Interactions2
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Fennel and Estradiol

These two interact in 2 distinct ways. Each one is explained separately below.

Why Are There 2 Interactions?

Fennel interacts with whole categories of medication. Estradiol falls into 2 of those categories, so it can interact with Fennel in 2 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Moderate).

Moderate

CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES

1 of 2

Why this applies to Estradiol

Estradiol falls under CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES — the group Fennel interacts with.

Theoretically, fennel might increase levels of drugs metabolized by CYP3A4.

In vitro research suggests that fennel inhibits CYP3A4 enzyme activity. This effect has not been reported in humans.
Severity Moderate Likelihood Possible Evidence D

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the gist for estradiol: your body uses an enzyme called CYP3A4 to help break it down and clear it out. Lab studies hint that fennel may slow that same enzyme. If that happened in people, estradiol could clear a little more slowly and build up, which in theory might mean stronger estrogen effects like breast tenderness, bloating, nausea, headaches, or breakthrough spotting.

The important part: this is only a test-tube finding, and it has not been seen in actual humans. Fennel used as a food or spice is very unlikely to matter. Concentrated fennel supplements or the essential oil are where I'd be a bit more cautious. If you use those regularly, just run it by your pharmacist or doctor before mixing the two.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Subehan, Usia, T., Iwata, H., Kadota, S., and Tezuka, Y. Mechanism-based inhibition of CYP3A4 and CYP2D6 by Indonesian medicinal plants. J Ethnopharmacol. 5-24-2006;105(3):449-455. PubMed
  2. Subehan, Zaidi, S. F., Kadota, S., and Tezuka, Y. Inhibition on human liver cytochrome P450 3A4 by constituents of fennel (Foeniculum vulgare): identification and characterization of a mechanism-based inactivator. J Agric.Food Chem. 12-12-2007;55(25):101 PubMed
Moderate

ESTROGENS

2 of 2

Estradiol also falls under ESTROGENS.

Theoretically, taking large amounts of fennel might interfere with hormone replacement therapy due to competition for estrogen receptors.

Some constituents of fennel have estrogenic activity.
Severity Moderate Likelihood Possible Evidence D

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the gist with fennel and your estradiol. Fennel contains plant compounds that act a little like estrogen in the body. The theory is that in large, concentrated amounts these compounds could compete with your estradiol at the same estrogen receptors, possibly nudging how well your hormone therapy works. This is purely a theoretical concern based on lab pharmacology, not real-world reports, so I don't want you to worry.

The everyday takeaway: enjoying fennel as a food or spice is very unlikely to cause any problem. The caution applies more to concentrated fennel supplements or fennel essential oil. If you're considering one of those regularly, just run it by your pharmacist or doctor first so they can keep an eye on your symptoms.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Leung AY, Foster S. Encyclopedia of Common Natural Ingredients Used in Food, Drugs and Cosmetics. 2nd ed. New York, NY: John Wiley & Sons, 1996.

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Fennel Also Interacts With Other Medications

Fennel has 739 moderate known drug interactions in our database — the Estradiol one above is among them. If you take anything else, check it against the full list below.

Moderate 739 No known interaction 2,091 (incl. Estradiol)
Taking anything else?

Check Another of Your Medications Against Fennel

Covers the 2,830 medications we’ve checked against Fennel. Pick one to open its full report.

From our Q&A

Questions About Fennel or Estradiol

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Fennel or Estradiol, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Estradiol

This page covers Fennel only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Estradiol page lists 213 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Grapefruit (major), St. John's Wort (major), Acacia Rigidula (moderate) and Adrafinil (moderate).

By severity: 3 major · 172 moderate · 38 minor.

From our pharmacist’s overview on the Estradiol page

“Estradiol (Estrace, Vivelle, Vagifem, and other brands) is estrogen therapy, and its interaction list deserves careful reading, especially the three entries rated major. Grapefruit can push estradiol levels higher, St.”

Reviewed by 2 · Jul 18, 2026

Open the Estradiol interaction page

One more smart check

Taking Fennel as Part of a Dietary Supplement Product? Check It for Interactions

We hold 2,219 real supplement products containing Fennel, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Fennel. Your product may carry other ingredients that interact with Estradiol too, on top of Fennel — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Fennel and Estradiol Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Fennel
Top 10%

#111 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Estradiol
Top 25%

#398 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About Fennel

What is Fennel?

Fennel
  • Digestive upset and bloating
  • Infant colic
  • Menstrual cramps
  • Cough and respiratory complaints
  • Breast milk support (traditional)

Fennel is a Mediterranean herb widely used as a food and spice, and traditionally taken for digestive complaints, colic, and menstrual cramps. Some small studies suggest possible benefit for certain stomach and menstrual symptoms, but the evidence is limited. It is generally well tolerated in food amounts, but concentrated supplements and the essential oil should be used with caution and after checking with your pharmacist or doctor.

Read the full Fennel monograph & all interactions
Keep checking

Other Drugs & Fennel

Browse every other medication we’ve checked against Fennel — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
739Moderate
0Minor
2,090No known interaction
739 Moderate 2,090 No interaction
Moderate

739 interactions

Showing 60 of 739 moderate interactions — see all 739 on the Fennel page.

No interaction 2,090 drugs checked — show the list

We also checked Fennel against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,090 medications checked with no known interaction — see the full Fennel list.

See the full Fennel interactions page
See every supplement checked against Estradiol, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Fennel monograph › Uses, safety, and every drug interaction in one place Estradiol supplement interactions › Every supplement checked against Estradiol, ranked by severity Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Fennel and Estradiol Together: Common Questions

Can you take Fennel with Estradiol?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
Why are there 2 interactions listed for Fennel and Estradiol?
Estradiol belongs to more than one drug category, and Fennel interacts with each of those categories a little differently. We list every one so nothing is missed — your overall risk is driven by the most serious of them.
How serious is the Fennel and Estradiol interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Estradiol and Fennel?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.