Drug & supplement interaction

Glucomannan + Venlafaxine

Venlafaxine brand names: Effexor XR, Effexor.

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

Glucomannan has a moderate interaction with Venlafaxine. Theoretically, glucomannan may decrease absorption of drugs taken orally.

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

Glucomannan and Venlafaxine

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

Moderate

ORAL DRUGS

Why this applies to Venlafaxine

Venlafaxine falls under ORAL DRUGS — the group Glucomannan interacts with.

Theoretically, glucomannan may decrease absorption of drugs taken orally.
Due to its viscosity and bulking effects, there is concern that glucomannan can decrease the absorption of oral drugs. A small clinical study in healthy volunteers shows that taking glyburide 2.5 mg plus glucomannan 3.9 grams with breakfast reduces plasma levels of glyburide when compared with breakfast and glyburide alone. In addition, animal research demonstrates this effect on amoxicillin, but shows increased absorption of metronidazole. This mouse model also demonstrates that metronidazole elimination is prolonged, but amoxicillin elimination is enhanced by 38%; glucomannan may also affect the distribution of some drugs. To avoid changes in absorption, take glucomannan 30-60 minutes after taking oral drugs.
Severity Moderate Likelihood Probable Evidence B

Interaction 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..

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

Here's the simple version: glucomannan is a thick, gel-forming fiber, and when it's swelling in your gut it can grab onto a pill like venlafaxine and slow or reduce how much your body absorbs. Venlafaxine isn't a prodrug, so less getting in usually means a weaker effect. Over time that could look like your depression or anxiety not being as well controlled as it was.

The evidence here is modest (mostly a small fiber study with a different drug plus some animal data), so this is more of a sensible precaution than a sure thing. An easy fix is timing: take your venlafaxine first, then wait about 30 to 60 minutes before the glucomannan. Before you combine them, run it by your pharmacist or doctor.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Shima K, Tanaka A, Ikegami H, et al. Effect of dietary fiber, glucomannan, on absorption of sulfonylurea in man. Horm Metab Res 1983;15:1-3. PubMed
  2. Zhang Y, Aldamarany WAS, Song G, et al. Influence of konjac glucomannan and its derivatives on the oral pharmacokinetics of antimicrobial agent in antibiotics cocktails: Keep vigilant on dietary fiber supplement. Int J Biol Macromol 2023;251:126306. 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

Glucomannan Also Interacts With Other Medications

Glucomannan has 2,021 moderate known drug interactions in our database — the Venlafaxine one above is one of them. If you take anything else, check it against the full list below.

Moderate 2,021 No known interaction 809 (incl. Venlafaxine)
Taking anything else?

Check Another of Your Medications Against Glucomannan

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

From our Q&A

Questions About Glucomannan or Venlafaxine

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Glucomannan or Venlafaxine, 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 Venlafaxine

This page covers Glucomannan only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Venlafaxine page lists 110 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), Arsenic (major), Cesium (major) and Ephedra (major).

By severity: 8 major · 82 moderate · 20 minor.

From our pharmacist’s overview on the Venlafaxine page

“Venlafaxine (Effexor, Effexor XR) is an antidepressant that raises serotonin, which is exactly where part of the danger on this page sits, and its 110 interactions deserve genuine respect. Eight carry a major rating, most tied to heart rhythm: several ingredients, grapefruit among them, can stretch the QT interval and add to a strain v…”

Reviewed by 2 · Jul 18, 2026

Open the Venlafaxine interaction page

One more smart check

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

We hold 467 real supplement products containing Glucomannan, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Glucomannan. Your product may carry other ingredients that interact with Venlafaxine too, on top of Glucomannan — 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 Glucomannan and Venlafaxine 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 Glucomannan
Top 25%

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

29 days ago today
Medication Venlafaxine
Top 5%

#107 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 Glucomannan

What is Glucomannan?

Glucomannan
  • Weight loss and appetite control
  • Constipation
  • High cholesterol
  • Blood sugar support
  • Digestive health

Glucomannan is a soluble fiber from the konjac root that swells into a gel in your stomach, which may help with mild weight loss, constipation, and modestly lowering cholesterol. It is generally well tolerated when taken with plenty of water, but it must be taken carefully because it can swell and cause choking or blockage if not swallowed properly. Talk with your pharmacist or doctor before starting it, especially if you have a health condition or take medicines.

Read the full Glucomannan monograph & all interactions
Medication

About Venlafaxine

What Is Venlafaxine Used For?

Brand names: Effexor XR, Effexor

Venlafaxine is used to treat major depressive disorder (MDD) across all available formulations. Extended-release capsules (including Effexor XR) are also approved for generalized anxiety disorder (GAD), social anxiety disorder (SAD), and panic disorder (PD).

Extended-release tablets are approved for major depressive disorder and social anxiety disorder. Immediate-release venlafaxine tablets are indicated for major depressive disorder only.

See full Venlafaxine drug information
Keep checking

Other Drugs & Glucomannan

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

0Major
2,021Moderate
0Minor
808No known interaction
2,021 Moderate 808 No interaction
Moderate

2,021 interactions

Showing 60 of 2,021 moderate interactions — see all 2,021 on the Glucomannan page.

No interaction 808 drugs checked — show the list

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

Showing the 60 most looked-up of 808 medications checked with no known interaction — see the full Glucomannan list.

See the full Glucomannan interactions page
See every supplement checked against Venlafaxine, 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
Glucomannan monograph › Uses, safety, and every drug interaction in one place Venlafaxine supplement interactions › Every supplement checked against Venlafaxine, ranked by severity Venlafaxine 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 Glucomannan and Venlafaxine Together: Common Questions

Can you take Glucomannan with Venlafaxine?
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 Glucomannan and Venlafaxine 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 Venlafaxine and Glucomannan?

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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.