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Metformin (Glucophage, Glumetza, Fortamet, Riomet) and Black patients

Brand names: Glucophage, Glumetza, Fortamet, Riomet

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What Metformin does

Metformin lowers blood glucose primarily by reducing the liver's glucose output and by modestly improving insulin sensitivity in peripheral tissues. It does not cause hypoglycemia on its own and is weight-neutral to slightly weight-reducing. It has been the first-line pharmacologic therapy for type 2 diabetes for over two decades.

What the evidence says for Black patients

Metformin works effectively across racial groups. The UKPDS, Diabetes Prevention Program (DPP), and subsequent trials found consistent A1C reductions and consistent cardiovascular effects in Black participants compared to white participants.

The 2024 ADA Standards of Care recommend metformin alongside — rather than strictly before — GLP-1 receptor agonists and SGLT2 inhibitors for high-risk patients. Black adults are disproportionately affected by diabetic kidney disease and heart failure, which are compelling indications to add (or prioritize) a GLP-1 or SGLT2 alongside metformin.

A critical dosing consideration specific to Black patients: the MDRD equation historically used a 'race coefficient' that over-estimated eGFR in Black adults. The 2021 CKD-EPI refit equation removed the coefficient, and many Black adults whose eGFR was previously reported as above 30 are now correctly below 30. Metformin is contraindicated below eGFR 30 and should be dose-reduced between 30 and 45. Patients who were borderline should have their metformin dose re-evaluated under the new eGFR equation — Inker et al., NEJM 2021;385:1737–49, PMID 34554658.

Metformin use during pregnancy is increasingly accepted for gestational diabetes, but Black women disproportionately affected by preeclampsia and preterm birth should discuss the trade-offs with a maternal-fetal-medicine clinician.

Common alternatives

When metformin is contraindicated or not tolerated: GLP-1 receptor agonists (semaglutide, dulaglutide, tirzepatide) and SGLT2 inhibitors (empagliflozin, dapagliflozin, canagliflozin) are both appropriate first-line for many patients, especially those with cardiovascular disease, heart failure, or CKD. Sulfonylureas (glipizide, glimepiride) are cheap but cause weight gain and hypoglycemia and have fallen out of favor as first alternatives.

Side effects

  • GI upset — diarrhea, nausea, abdominal cramping — most common in the first 2–4 weeks. Extended-release forms reduce this.
  • Metallic taste
  • Vitamin B12 deficiency with long-term use — check levels periodically
  • Lactic acidosis — rare but serious; hold the drug during hospitalization, contrast imaging, or acute illness with dehydration

Factors that affect adherence

GI side effects drive early discontinuation; start at 500 mg once daily and uptitrate weekly. Generic metformin is on every pharmacy's $4 list. Taking with meals dramatically reduces GI symptoms.

Questions to ask your doctor

Bring this list to your next appointment.

  • Given the new 2021 eGFR equation, is my kidney function still above the cutoff for my current metformin dose?
  • If I have diabetic kidney disease or heart failure, should I also be on an SGLT2 or GLP-1?
  • When will we check my B12?
  • Should I stop this before any contrast imaging?

References

  1. American Diabetes Association. 9. Pharmacologic approaches to glycemic treatment: Standards of Care in Diabetes—2024. Diabetes Care. 2024;47(Suppl 1):S158–S178.
  2. Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. NEJM. 2021;385:1737–1749. PMID 34554658.
  3. UK Prospective Diabetes Study (UKPDS) Group. Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). Lancet. 1998;352:854–865.
  4. U.S. Food and Drug Administration. Glucophage (metformin hydrochloride) label. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/020357s037s039,021202s021s023lbl.pdf

Medical disclaimer

This page is patient education, not prescribing guidance. It summarizes the published evidence about how this medication has been studied in Black patients — it is not a substitute for the judgment of your personal clinician. Never start, stop, or change a prescription based on something you read here. If you have questions about your medication, call your prescriber or pharmacist. For emergencies, call 911.

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