Medication · biguanide

Supplements and Metformin.

Every documented pair, every citation. Below: 2 documented pairs grouped by mechanism.

Metformin, sold under the brand name Glucophage, is the first-line oral antidiabetic agent. It reduces hepatic glucose production and improves peripheral insulin sensitivity.

Below are the 2 documented pairs we have explicitly assessed against Metformin in the Distil database: 2 amber. The pairs cluster around 2 mechanisms: Absorption interference and Additive glucose lowering. Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not on this list is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by supplement, and the missing-item form at the bottom of the page routes any uncatalogued supplement into our next curation pass.

Documented interactions

Absorption interference

Amber Vitamin B12

Long-term metformin reduces vitamin B12 absorption in 10 to 30 percent of patients. After a year or more of daily use, ask your GP about an annual B12 blood test. Supplemental B12 (1000 mcg cyanocobalamin daily, or higher doses less frequently) reverses the deficiency in most people.

PMID 20488910 · PMID 26900641

Additive glucose lowering

Amber Berberine

Berberine and metformin both lower blood sugar through similar pathways. The combination can work but needs careful glucose monitoring, especially in the first few weeks, to avoid hypoglycaemia.

PMID 18397984 · BNF: Metformin

What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope (food-supplement interactions only; for drug-drug interactions, the BNF is authoritative). Use the checker below to surface any supplement, and submit a missing item if you take something we have not catalogued.

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For adults over 18. This tool gives evidence-graded information, not medical advice. Always discuss changes with your GP, especially if you take any medication, are pregnant, breastfeeding, or have a serious health condition.

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Your supplements

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Your medications

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How we decide

How we grade severity, choose what's in scope, and what we exclude.

Every call on this page is reasoned. We publish the full rubric for severity tiers, the medication inclusion logic, the evidence grades we accept, and what we deliberately leave out. About three thousand words. Worth reading once if you use this tool more than occasionally.

Read the full methodology

Something missing?

If a supplement or medication you take isn't in our autocomplete, tell us and we'll add it in the next quarterly update.

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Distil's interactions database is reviewed and updated every quarter. We grade evidence transparently and publish our methodology, including every database change, at /about/methodology. This tool is information, not a substitute for clinical judgement. If you take medication and supplements together, your GP or pharmacist can review your full regimen against your medical history. If you want a comprehensive personalised stack reasoned against this same database, the Distil report is the next step up.