Drug & supplement interaction

Fennel + Progestin, Oral Contraceptive

Progestin, Oral Contraceptive brand name: Slynd.

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

Fennel has a moderate interaction with Progestin, Oral Contraceptive. Theoretically, fennel might increase levels of drugs metabolized by CYP3A4.

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

Fennel and Progestin, Oral Contraceptive

Here is the interaction we found between these two, with the evidence behind it.

Moderate

CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES

Why this applies to Progestin, Oral Contraceptive

Progestin, Oral Contraceptive 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

Your birth control pill Slynd (drospirenone) is cleared from your body partly by a liver enzyme called CYP3A4. In test-tube studies, fennel appeared to slow that enzyme down a bit. If that happened in real life, your progestin could build up slightly, which in theory might mean a little more chance of things like breakouts, breast tenderness, headaches, mood changes, or spotting.

Here's the reassuring part: this was only seen in lab dishes, never in actual people, so it's mostly theoretical. The amount of fennel you'd get in cooking or tea is very unlikely to matter. Concentrated fennel supplements or the essential oil are where I'd be more cautious. If you're using those regularly, just give your pharmacist a quick heads-up.

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

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 Progestin, Oral Contraceptive one above is one of them. If you take anything else, check it against the full list below.

Moderate 739 No known interaction 2,091 (incl. Progestin, Oral Contraceptive)
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 Progestin, Oral Contraceptive

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 Progestin, Oral Contraceptive, 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 Progestin, Oral Contraceptive

This page covers Fennel only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Progestin, Oral Contraceptive page lists 139 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 · 95 moderate · 41 minor.

From our pharmacist’s overview on the Progestin, Oral Contraceptive page

“If you take Slynd, a progestin-only birth control pill, treat this list with real care, because anything that lowers the hormone level can weaken protection against pregnancy. Slynd is cleared by the CYP3A4 enzyme, and nearly all 139 ingredients on file touch that pathway one way or another.”

Reviewed by 2 · Jul 18, 2026

Open the Progestin, Oral Contraceptive 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 Progestin, Oral Contraceptive 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 Progestin, Oral Contraceptive 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%

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

29 days ago today
Medication Progestin, Oral Contraceptive
Top 10%

#222 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
Medication

About Progestin-Only (drospirenone) Oral Contraceptives

What Is Progestin-Only (drospirenone) Oral Contraceptives Used For?

Brand name: Slynd

  • Progestin-only (drospirenone) oral contraceptives are used to prevent pregnancy. Progestin is a female hormone. It works by preventing the release of eggs from the ovaries (ovulation) and changing the cervical mucus and the lining of the uterus. Progestin-only oral contraceptives are a very effective method of birth control, but they do not prevent the spread of of human immunodeficiency virus (HIV, the virus that causes acquired immunodeficiency syndrome [AIDS]) and other sexually transmitted diseases.

See full Progestin-Only (drospirenone) Oral Contraceptives drug information
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 Progestin, Oral Contraceptive, 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 Progestin, Oral Contraceptive supplement interactions › Every supplement checked against Progestin, Oral Contraceptive, ranked by severity Progestin-Only (drospirenone) Oral Contraceptives drug information › The full plain-English guide to this medication 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 Progestin, Oral Contraceptive Together: Common Questions

Can you take Fennel with Progestin, Oral Contraceptive?
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.
How serious is the Fennel and Progestin, Oral Contraceptive 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 Progestin, Oral Contraceptive 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.