Drug & supplement interaction

Fennel + Caffeine (prescription drug)

Caffeine (prescription drug) brand names: Dextrophin, Caffeine.

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

Fennel has a moderate interaction with Caffeine (prescription drug). Theoretically, fennel might increase levels of drugs metabolized by CYP3A4.

Read the full interaction report
SeverityModerate
LikelihoodPossible
EvidenceD
What do Severity, Likelihood & Evidence mean?
Know someone taking this? Share
Facebook Post Reddit Email
On this page
Interaction report

Fennel and Caffeine (prescription drug)

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

Moderate

CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES

Why this applies to Caffeine (prescription drug)

Caffeine (prescription drug) 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 short version: the concern on file is that fennel might slow down a liver enzyme called CYP3A4, which can let certain drugs build up in your body. With caffeine, though, I'd take this one with a big grain of salt. Caffeine isn't really a prodrug, and it's mainly handled by a different enzyme (CYP1A2), not CYP3A4, so this particular pathway plays only a minor role for it.

On top of that, the worry comes from lab dish studies, not from anything seen in actual people. Everyday culinary fennel in food is very unlikely to matter. If you're using concentrated fennel supplements or the essential oil and feel extra jittery, restless, or notice a racing heart, check in with your pharmacist or doctor.

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 741 moderate known drug interactions in our database — the Caffeine (prescription drug) one above is one of them. If you take anything else, check it against the full list below.

Moderate 741 No known interaction 2,089 (incl. Caffeine (prescription drug))
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 Caffeine (prescription drug)

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 Caffeine (prescription drug), 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 Caffeine (prescription drug)

This page covers Fennel only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Caffeine (prescription drug) page lists 208 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Ephedra (major), Grapefruit (major), Peyote (major) and Rauvolfia Vomitoria (major).

By severity: 7 major · 155 moderate · 46 minor.

From our pharmacist’s overview on the Caffeine (prescription drug) page

“Prescription caffeine is itself a stimulant, which is exactly why this list calls for genuine care rather than a quick skim. Seven of the 208 ingredients on file are rated major, and those deserve attention first.”

Reviewed by 2 · Jul 18, 2026

Open the Caffeine (prescription drug) 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 Caffeine (prescription drug) 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 Caffeine (prescription drug) 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%

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

29 days ago today
Medication Caffeine (prescription drug)
Top 5%

#121 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 Caffeine

What Is Caffeine Used For?

Brand names: Dextrophin, Caffeine

Caffeine citrate injection and oral solution are used for the short-term treatment of apnea of prematurity. This means pauses in breathing in premature infants. The label lists the age range as infants between 28 weeks and a later gestational age, and that range is cut off in the text provided.

Over-the-counter caffeine tablets, such as Diurex and the Stay Awake products, are taken by mouth by adults and older children. Their labels point to fatigue and drowsiness. Diurex directions also mention taking it while symptoms persist.

See full Caffeine 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
741Moderate
0Minor
2,088No known interaction
741 Moderate 2,088 No interaction
Moderate

741 interactions

Showing 60 of 741 moderate interactions — see all 741 on the Fennel page.

No interaction 2,088 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 2,088 medications checked with no known interaction — see the full Fennel list.

See the full Fennel interactions page
See every supplement checked against Caffeine (prescription drug), 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 Caffeine (prescription drug) supplement interactions › Every supplement checked against Caffeine (prescription drug), ranked by severity Caffeine 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 Caffeine (prescription drug) Together: Common Questions

Can you take Fennel with Caffeine (prescription drug)?
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 Caffeine (prescription drug) 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 Caffeine (prescription drug) and Fennel?

Our pharmacists answer your medication & supplement questions — free.

Ask a pharmacist

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.