Drug & supplement interaction

Theaflavin + Metformin Hydrochloride

Metformin Hydrochloride brand name: Riomet ER.

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

Theaflavin has a moderate interaction with Metformin Hydrochloride. In an animal diabetic model, giving theaflavin reduced fasting blood glucose, with an effect comparable to metformin.

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

Theaflavin and Metformin Hydrochloride

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

Moderate

ANTIDIABETES DRUGS

Why this applies to Metformin Hydrochloride

Metformin Hydrochloride falls under ANTIDIABETES DRUGS — the group Theaflavin interacts with.

In an animal diabetic model, giving theaflavin reduced fasting blood glucose, with an effect comparable to metformin. A fermented green tea product containing theaflavin also appears to reduce blood glucose levels in healthy humans and in an animal model. Theoretically, concomitant use with antidiabetes drugs might cause additive hypoglycemic effects. Use with caution.
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 simple version: theaflavin (the antioxidant in black tea) may have a mild blood-sugar-lowering effect of its own. Metformin already works to bring your glucose down, so stacking the two together could, in theory, push your sugar a little lower than expected. This isn't about theaflavin changing how your body processes metformin. It's just two things nudging in the same direction at once.

Honestly, the evidence here is pretty thin (mostly animal studies and lab work), and a normal cup or two of black tea is very unlikely to cause trouble. The bigger question mark is with concentrated theaflavin extracts. If you take those, keep an eye out for low-sugar signs like shakiness, sweating, or dizziness, check your readings, and chat with your pharmacist or doctor before adding the supplement.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Takemoto M, Takemoto H, Kouno H, Soeda T, Moriya T. A simple, enzymatic biotransformation method using fresh green tea leaves efficiently generates theaflavin-containing fermentation water that has potent physiological functions in mice and humans. Biol P PubMed
  2. Gothandam K, Ganesan VS, Ayyasamy T, Ramalingam S. Antioxidant potential of theaflavin ameliorates the activities of key enzymes of glucose metabolism in high fat diet and streptozotocin - induced diabetic rats. Redox Rep. 2019;24(1):41-50. 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

Theaflavin Also Interacts With Other Medications

Theaflavin has 129 moderate known drug interactions in our database — the Metformin Hydrochloride one above is one of them. If you take anything else, check it against the full list below.

Moderate 129 No known interaction 2,701 (incl. Metformin Hydrochloride)
Taking anything else?

Check Another of Your Medications Against Theaflavin

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

Everything else you take

See Every Supplement That Interacts With Metformin Hydrochloride

This page covers Theaflavin only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Metformin Hydrochloride page lists 257 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Siberian Cocklebur (major), Acai (moderate), African Wild Potato (moderate) and Agaricus Mushroom (moderate).

By severity: 2 major · 217 moderate · 38 minor.

From our pharmacist’s overview on the Metformin Hydrochloride page

“Anyone taking metformin hydrochloride (Riomet ER) should treat this list with genuine care, because two of its 257 entries are rated major. Siberian cocklebur has caused severe hypoglycemia in people and should not be combined with metformin at all, and activated charcoal can adsorb the drug in the gut so less reaches your blood.”

Reviewed by 2 · Jul 18, 2026

Open the Metformin Hydrochloride interaction page

One more smart check

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

We hold 11 real supplement products containing Theaflavin, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Theaflavin. Your product may carry other ingredients that interact with Metformin Hydrochloride too, on top of Theaflavin — 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 Theaflavin and Metformin Hydrochloride 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 Theaflavin
Top half

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

29 days ago today
Medication Metformin Hydrochloride
Top 10%

#241 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 Theaflavin

What is Theaflavin?

  • Heart health and cholesterol support
  • Antioxidant support
  • Weight management
  • Blood sugar support
  • General wellness

Theaflavins are antioxidant compounds formed when tea leaves are oxidized to make black tea. Early research suggests they may help with cholesterol and provide antioxidant effects, but high-quality human evidence is limited and doses are not standardized. Drinking black tea is generally safe, but talk to your pharmacist or doctor before taking concentrated extracts.

Read the full Theaflavin monograph & all interactions
Keep checking

Other Drugs & Theaflavin

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

0Major
129Moderate
0Minor
2,700No known interaction
129 Moderate 2,700 No interaction
Moderate

129 interactions

Showing 60 of 129 moderate interactions — see all 129 on the Theaflavin page.

No interaction 2,700 drugs checked — show the list

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

Showing the 60 most looked-up of 2,700 medications checked with no known interaction — see the full Theaflavin list.

See the full Theaflavin interactions page
See every supplement checked against Metformin Hydrochloride, 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
Theaflavin monograph › Uses, safety, and every drug interaction in one place Metformin Hydrochloride supplement interactions › Every supplement checked against Metformin Hydrochloride, 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 Theaflavin and Metformin Hydrochloride Together: Common Questions

Can you take Theaflavin with Metformin Hydrochloride?
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 Theaflavin and Metformin Hydrochloride 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 Metformin Hydrochloride and Theaflavin?

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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.