Nutrients and interactions
Supplements if you take metformin
Long-term metformin more than doubles the odds of vitamin B12 deficiency in meta-analyses, and the risk grows with the dose and the years of use, so B12 is the one nutrient that clearly matters here. Picks are ranked by how strong the evidence is that metformin lowers the nutrient or that the supplement helps a problem common on the drug. This page is about nutrients and interactions: never stop or change metformin without your prescriber.
This page covers nutrients and supplement interactions. Never stop or change a prescribed medication without your prescriber, and check any new supplement with your pharmacist.
Evidence last reviewed: October 2026
Top picks
| Option | Evidence | Best for | The honest note | |
|---|---|---|---|---|
| Vitamin B12 | Strong evidence | long-term metformin users | Metformin users had 2.45 times the odds of B12 deficiency in a 29-study meta-analysis, and oral B12 reliably corrects deficiency. Test first, take 250–1,000 mcg a day if your level is low, and recheck with your doctor. | View |
Solid options
| Option | Evidence | Best for | The honest note | |
|---|---|---|---|---|
| Methylcobalamin | Moderate evidence | same job, higher price | It corrects deficiency like any oral B12, at 1,000–2,000 mcg a day when you are low, but there is no evidence it beats cheaper cyanocobalamin. Choose it for preference, not because metformin users need the methylated form. | View |
Situational
| Option | Evidence | Best for | The honest note | |
|---|---|---|---|---|
| Alpha-Lipoic Acid | Moderate evidence | diabetic nerve pain, after B12 testing | At 600 mg a day it eased neuropathy symptoms more than placebo in a 181-patient trial. Numbness on metformin can also come from B12 deficiency, so have B12 checked first, and watch for low glucose if you also use insulin or a sulfonylurea. | View |
| Benfotiamine | Moderate evidence | neuropathy symptoms, at 600 mg | In a 6-week trial only 600 mg a day, not 300 mg, improved neuropathy symptom scores, and no trial shows better nerve conduction. It does not lower glucose, and self-treating numbness can delay finding a B12 deficiency. | View |
Skip, or avoid with this medication
- Berberine Moderate evidence
It adds to metformin's glucose lowering with a risk of hypoglycemia and raises metformin exposure, and head-to-head trials found it no better than metformin itself.
- Dihydroberberine Limited evidence
It is better absorbed than berberine, but its only human trial found no improvement in glucose response, and it carries the same additive glucose-lowering risk.
- Blood Sugar Support Formulas Limited evidence
Most contain a fraction of the studied doses, the blends are rarely tested as sold, and none of them replaces prescribed treatment.
- Cassia Cinnamon Moderate evidence
It lowers fasting glucose a little with an unclear effect on HbA1c, adds to glucose-lowering drugs, and its coumarin content limits how much you can safely take.
- CLA (Conjugated Linoleic Acid) Limited evidence
The trans-10,cis-12 isomer worsens insulin sensitivity, which works against your diabetes treatment.
- Guar Gum Moderate evidence
It delays metformin absorption, so if you use it for glucose control, keep it at least 2 hours away from your dose.
Our verdict
Ask for a B12 level if you have taken metformin for years, and if it is low take 250–1,000 mcg a day of any B12 form; plain cyanocobalamin works as well as methylcobalamin. Skip berberine and blood sugar blends on top of metformin, since they add to its glucose lowering and berberine also raises metformin exposure.