Glucomannan and Metformin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Metformin
Glucomannan
No brand names on record
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Increased risk of hypoglycemia
Interaction Deep Dive
High-fiber or fiber-supplemented diets are recommended to patients with diabetes mellitus to decrease postprandial hyperglycemia, hence guar gum, pectin, and glucomannan have found increased use12. Glucomannan, an unabsorbable polysaccharide, has been shown to decrease glyburide plasma concentrations by 50% 1. Glucose concentrations were found to be decreased despite the reduced absorption of glyburide 1. Insulin and hypoglycemic agent dose requirements were reduced in patients taking glucomannan 2. Glucomannan may slow gastric emptying, increase the viscosity of gastrointestinal contents, and act as a barrier to diffusion, which slows absorption of hypoglycemic agents and glucose 12. Blood glucose should be monitored closely in patients taking glucomannan concomitantly with antidiabetic agents.
Why it happens (mechanism)
Additive hypoglycemic effect
Literature reports
2 reports — tap to read
a) Coadministration of glucomannan and glibenclamide (glyburide) decreased glibenclamide concentration in 9 healthy male volunteers aged 21 to 47 years. Glibenclamide 2.5 milligrams (mg) was administered on day 1 without glucomannan, followed by concomitant administration with glucomannan 3.9 grams on day 2. Plasma glibenclamide concentrations were reduced by approximately 50% when coadministered with glucomannan. Glibenclamide plasma concentrations with glucomannan versus control were 31.7 +/- 24.5 ng/mL versus 76.4 +/- 25.0 ng/mL at 30 minutes (p less than 0.002); 51.3 +/- 35.5 ng/mL versus 120.9 +/- 56.0 ng/mL at 60 minutes (p less than 0.02); 60.0 +/- 38.8 ng/mL versus 117.4 +/- 53.1 ng/mL at 90 minutes (p less than 0.05), and 54 +/- 31.5 ng/mL versus 100.7 +/- 46.5 ng/mL at 150 minutes (p less than 0.005). Interestingly, plasma glucose concentrations were lower at 30 minutes in those receiving glibenclamide with glucomannan than in those receiving glibenclamide with control 1.
b) In 13 patients with diabetes supplemented with 3.6 grams or 7.2 grams glucomannan (konjac mannan) for 90 days, mean fasting blood glucose was reduced by 29% (p less than 0.025). Dose requirements of insulin or hypoglycemic agents were reduced, and withdrawn in some patients 2.
Common questions
Can I take Glucomannan and Metformin together?
Increased risk of hypoglycemia Always confirm with your pharmacist or prescriber before making any change.
How serious is the Glucomannan and Metformin interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Glucomannan or Metformin need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (2)
Keep reading about Metformin
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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