Metformin: Answers to the Most Frequently Asked Questions

What Is Metformin Used For?

Metformin (brand name [Glucophage](/drugs/metformin)) is the most commonly prescribed medication for [type 2 diabetes](/conditions/type-2-diabetes) worldwide. It is FDA-approved as a first-line treatment for type 2 diabetes in adults and children over 10 years old. It is also used off-label for:

Prediabetes: The Diabetes Prevention Program showed metformin reduced progression from prediabetes to type 2 diabetes by 31%.
Polycystic ovary syndrome (PCOS): Improves insulin resistance, menstrual regularity, and ovulation.
Weight management: Causes modest weight loss (2–3 kg on average) as a secondary effect.

How Does Metformin Work?

Metformin primarily works by reducing glucose production in the liver (hepatic gluconeogenesis). The liver of a person with type 2 diabetes produces too much glucose, especially overnight — metformin suppresses this. It also improves insulin sensitivity in muscle cells, allowing them to take up glucose more effectively.

Metformin activates AMP-activated protein kinase (AMPK), a cellular energy sensor. This is thought to be central to many of its metabolic effects, including reduced hepatic glucose output and improved insulin sensitivity.

Does Metformin Cause Lactic Acidosis?

Lactic acidosis is a rare but serious complication associated with metformin — but the risk is extremely low in patients with normal kidney function. The estimated incidence is approximately 3 cases per 100,000 patient-years, which is comparable to the background rate of lactic acidosis in the general population.

The risk is primarily a concern in patients with:

Severe kidney impairment (eGFR <30 mL/min) — metformin is contraindicated
Liver disease or heavy alcohol use
Acute illness causing dehydration or reduced kidney perfusion
Upcoming procedures using iodinated contrast dye (hold metformin before and 48 hours after)

For patients with normal kidney function, the risk of lactic acidosis is negligible and should not be a reason to avoid metformin.

Does Metformin Cause Vitamin B12 Deficiency?

Yes — this is a real and underappreciated side effect. Long-term metformin use reduces vitamin B12 absorption in the gut by approximately 30%. Studies show that 5–10% of long-term metformin users develop B12 deficiency, and up to 30% have borderline-low levels.

B12 deficiency can cause peripheral neuropathy (numbness and tingling in the hands and feet) — a symptom that can be mistakenly attributed to diabetic neuropathy. The American Diabetes Association recommends periodic B12 monitoring in patients on long-term metformin, especially those with symptoms of neuropathy or anemia. B12 supplementation (500–1,000 mcg daily) is inexpensive and effective.

What Are the Most Common Side Effects?

Gastrointestinal side effects affect up to 30% of patients and include nausea, diarrhea, abdominal cramping, and a metallic taste. These are usually transient and can be minimized by:

Starting at a low dose (500 mg once daily with dinner) and titrating slowly over 4–8 weeks
Taking metformin with food
Switching to the extended-release formulation (metformin ER/XR), which has significantly fewer GI side effects

Unlike many diabetes medications, metformin does not cause hypoglycemia (low blood sugar) when used alone, because it does not stimulate insulin secretion.

Does Metformin Cause Weight Gain?

No — metformin is weight-neutral to modestly weight-reducing. Most patients lose 1–3 kg (2–6 lbs) over the first year of treatment. This is one of its advantages over sulfonylureas (which cause weight gain) and insulin (which causes significant weight gain).

Can I Drink Alcohol While Taking Metformin?

Moderate alcohol consumption is generally acceptable, but heavy or binge drinking should be avoided. Alcohol increases the risk of lactic acidosis by impairing liver function and increasing lactate production. The combination of metformin and heavy alcohol use is one of the main risk factors for metformin-associated lactic acidosis.

How Much Does Metformin Cost Without Insurance?

Metformin is one of the least expensive medications available. Generic metformin typically costs $4–$15 for a 30-day supply. Metformin ER (extended-release) costs slightly more at $10–$25/month. Walmart's $4 prescription program includes metformin immediate-release.

References

American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1).
Diabetes Prevention Program Research Group. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393-403.
Aroda VR, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754-1761.