Supplements and Metformin.
Metformin, sold under the brand name Glucophage, is the first-line oral antidiabetic agent. It reduces hepatic glucose production and improves peripheral insulin sensitivity.
Metformin is the first line oral medicine for type 2 diabetes. It is the most prescribed antidiabetic agent in the UK by a wide margin. It reduces hepatic glucose production through AMPK activation and improves peripheral insulin sensitivity, without causing hypoglycaemia on its own. The most documented effect on the supplement side is vitamin B12 depletion. Long use lowers serum B12 in 10 to 30 percent of patients per the DPPOS extension data, and NICE NG28 recommends annual B12 screening on patients who have been on metformin for years. The mechanism is intestinal. Metformin interferes with the step of B12 absorption that depends on calcium, in the terminal ileum. Berberine carries the closest mechanism overlap (it also activates AMPK) and the combination has been studied. The picture is additive HbA1c reduction at the cost of additive GI upset. Standard metformin GI side effects (nausea, loose stools) settle in most patients within two to four weeks of starting or titrating up. The modified release formulation reduces them further.
Below are the 18 documented pairs we have explicitly assessed against Metformin in the Distil database: 17 amber and 1 green. The pairs cluster around 4 mechanisms: Absorption interference, Reduced glucose control, Additive glucose lowering, and Gut microbiome synergy. 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
Long-term metformin reduces how well your body absorbs vitamin B12, and the B12 in your B-complex is the part that matters here. This affects 10 to 30 percent of people on metformin. Many standard B-complexes contain less B12 than the amount shown to reverse this (around 1000 mcg daily), so a dedicated higher-dose B12 may be needed. After a year or more of daily metformin, ask your GP about an annual B12 blood test, because absorption can stay impaired even while supplementing.
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.
Reduced glucose control
Melatonin can blunt the body's insulin response and raise blood sugar after eating, which works against the blood-sugar control that metformin is helping with. The effect is strongest when melatonin and food are taken close together, so if you take metformin it is best to keep melatonin well away from your evening meal and to mention it to your prescriber if your blood sugar is being monitored.
Additive glucose lowering
Alpha lipoic acid may improve how your body responds to insulin and lower blood sugar. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable because metformin on its own rarely causes low blood sugar, but monitor your glucose when you start alpha lipoic acid or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
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.
Bergamot can lower blood sugar on its own, and metformin also lowers it, so the two may add up. The combination is generally manageable because metformin rarely causes very low glucose by itself, but monitor your blood sugar when you start bergamot and tell your GP.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Oat beta-glucan modestly lowers blood sugar on its own, so taking it with metformin adds a little to metformin's effect. Metformin rarely causes low blood sugar by itself, so the practical risk is small, but it is worth keeping an eye on your glucose when you first add regular beta-glucan.
Chromium may improve how your body responds to insulin and lower blood sugar. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable, but monitor your blood glucose, especially when you start chromium or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Cinnamon may lower blood sugar a little on its own, though the evidence is mixed. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable because metformin on its own rarely causes low blood sugar, but monitor your glucose when you start cinnamon or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Garlic supplements may lower blood sugar a little on their own. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable because metformin on its own rarely causes low blood sugar, but monitor your glucose when you start a garlic supplement or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Holy basil (tulsi) may lower blood sugar on its own. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable because metformin on its own rarely causes low blood sugar, but monitor your glucose when you start holy basil or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Myo-inositol may improve how your body responds to insulin and gently lower blood sugar. Metformin also lowers blood sugar, so taking the two together may add up. The combination is generally manageable and is sometimes used on purpose, but it is sensible to monitor your blood glucose when you start myo-inositol or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Ginseng can lower blood sugar a little, and metformin lowers it too, so the two may add up. Metformin on its own rarely causes hypos, so the combination is generally low risk, but it is still worth monitoring your blood sugar when you start ginseng or change the dose.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Pine bark extract (pycnogenol) may lower blood sugar a little on its own. Metformin also lowers blood sugar, so the two may add up. The combination is generally manageable because metformin on its own rarely causes low blood sugar, but monitor your glucose when you start pine bark extract or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Psyllium lowers blood sugar a little on its own, so taking it with metformin adds modestly to metformin's effect. Metformin rarely causes low blood sugar by itself, so the practical risk is small, but it is worth keeping an eye on your glucose when you first add regular psyllium. Some people also find psyllium settles metformin's stomach side effects.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Saffron can modestly lower blood sugar, and metformin lowers blood sugar too, so the two may add up. The combination is generally manageable, but monitor your blood glucose when you start saffron or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Milk thistle (silymarin) may lower blood sugar, and metformin also lowers it, so the two may add up. The combination is generally manageable because metformin rarely causes low blood sugar on its own, but monitor your blood glucose when you start silymarin or change the dose, and your doctor may adjust the metformin if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Gut microbiome synergy
These work in the same direction. Metformin itself tends to increase Akkermansia in the gut, so taking the supplement alongside it is complementary rather than a problem. No special timing is needed.
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.
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.
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