Metformin

is prescribed to treat:

• Type 2 diabetes and prediabetes • Various related conditions in the diabetes category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Metformin (Glucophage, Glumetza) is a biguanide and the first-line pharmacologic therapy for type 2 diabetes in nearly every major guideline worldwide. It has been in clinical use for more than sixty years, is inexpensive as a generic, does not cause weight gain, and — used on its own — does not cause hypoglycemia. Those properties explain its durability: newer drug classes have been added around metformin rather than replacing it.

Mechanism of Action

Metformin's dominant effect is on the liver. It suppresses hepatic gluconeogenesis, the process by which the liver manufactures glucose and releases it into the bloodstream, which is the main driver of elevated fasting blood sugar in type 2 diabetes. It works partly through activation of AMP-activated protein kinase (AMPK), a cellular energy sensor, and partly through AMPK-independent effects on the mitochondrial respiratory chain and on hepatic redox state.

Secondary effects contribute meaningfully. Metformin improves insulin sensitivity in skeletal muscle, so the glucose that is present is taken up more readily. It modestly slows intestinal glucose absorption, and it alters the gut microbiome and bile acid signalling in ways that appear to matter more than was once appreciated — which is part of why the extended-release formulation, despite lower peak plasma levels, works about as well as the immediate-release form.

Crucially, metformin does not stimulate insulin secretion. This is the mechanistic reason it does not cause hypoglycemia as monotherapy, and it is what separates metformin from sulfonylureas such as glipizide and glimepiride. Typical glycemic effect is a reduction in hemoglobin A1c of roughly 1 to 1.5 percentage points, with the largest reductions in patients starting from the highest baseline.

Clinical Use

Metformin is indicated for type 2 diabetes and is widely used in prediabetes, where the Diabetes Prevention Program demonstrated that it reduces progression to diabetes — though less effectively than intensive lifestyle change in the trial overall, and most effectively in younger patients, those with higher BMI, and women with a history of gestational diabetes. It is also a mainstay in polycystic ovary syndrome, where insulin resistance drives much of the metabolic and reproductive picture.

Where metformin sits in the sequence has shifted. It remains the usual first agent for glycemic control, but for patients with established cardiovascular disease, heart failure, or chronic kidney disease, guidelines now recommend adding an SGLT2 inhibitor such as empagliflozin or a GLP-1 receptor agonist such as semaglutide for their organ-protective benefits, independent of A1c. Metformin is generally continued alongside them rather than stopped. The American Diabetes Association publishes patient-level guidance on how these agents fit together.

The most common reason patients abandon metformin is gastrointestinal intolerance — nausea, cramping, and diarrhea. This is largely dose-related and largely avoidable: starting low, taking it with food, titrating slowly over several weeks, and switching to the extended-release formulation resolves the great majority of cases. Patients who were told years ago that they "could not tolerate metformin" after an abrupt full-dose start frequently tolerate a careful re-introduction.

Monitoring and Follow-Up

Kidney function governs metformin safety, because the drug is cleared unchanged by the kidneys and accumulation is what creates risk. Check eGFR before starting and at least annually, more often in older adults or anyone with reduced function. Metformin is contraindicated below an eGFR of 30 mL/min/1.73m². Between 30 and 45 it is not automatically stopped, but starting is not recommended and continuing calls for dose reduction and closer monitoring.

Hemoglobin A1c is checked roughly every three months until stable and every six months thereafter. Vitamin B12 deserves specific attention: long-term metformin use lowers B12 absorption, and clinically meaningful B12 deficiency develops in a substantial minority of long-term users. Because the neurologic symptoms of B12 deficiency can mimic diabetic neuropathy, it is worth measuring periodically rather than assuming numbness is nerve damage from diabetes itself. Our endocrine team monitors both alongside routine diabetes care, and the understanding your A1C article explains what the numbers mean in practice.

Weight is worth setting expectations about. Metformin is weight-neutral to modestly weight-reducing, typically a few pounds, and it is not a weight-loss drug — patients who expect substantial loss are usually disappointed and stop. Where weight is a primary target alongside glycemic control, a GLP-1 receptor agonist is the agent that addresses both. The MedlinePlus metformin entry sets out the full prescribing detail.

Lactic acidosis is the serious adverse effect metformin is known for, and it is genuinely rare — it occurs almost exclusively when the drug accumulates in the setting of acute kidney injury, severe dehydration, sepsis, decompensated heart failure, or heavy alcohol use. That risk profile is why metformin is held around procedures involving iodinated contrast and during acute illness with vomiting or dehydration.

Special Populations

In pregnancy, metformin crosses the placenta but has a reassuring accumulated safety record and is used in gestational diabetes and in PCOS, though insulin remains the reference standard when pharmacologic treatment is needed. In older adults, the drug is well tolerated but kidney function declines with age even when creatinine looks normal, so eGFR — not creatinine alone — should guide dosing. Patients with significant liver disease or heavy alcohol intake face higher lactic acidosis risk and generally need a different agent.

When to Contact Your Doctor

Seek urgent care for the combination of unusual muscle pain, difficulty breathing, unusual sleepiness, severe abdominal discomfort, and feeling cold — this cluster, particularly during an acute illness, is the presentation of lactic acidosis. Contact the office if gastrointestinal symptoms persist beyond a few weeks rather than stopping on your own, since a formulation change usually solves it. Report any illness causing vomiting, diarrhea, or reduced fluid intake, and mention metformin before any scheduled imaging with contrast or surgery.

If you are managing type 2 diabetes or prediabetes, contact us or schedule a visit to review your A1c, kidney function, and whether your current regimen still matches your cardiovascular and kidney risk.