Metformin ER 500 mg is primarily prescribed to lower blood sugar in adults with type 2 diabetes. The “ER” stands for extended release, meaning the tablet dissolves slowly throughout the day instead of all at once. It’s one of the most commonly prescribed medications in the world, and while blood sugar control is its main job, doctors also prescribe it for several other conditions.
Type 2 Diabetes: The Primary Use
Metformin ER is FDA-approved to improve blood sugar control in adults with type 2 diabetes, used alongside diet and exercise. The goal is to bring both fasting blood sugar and A1c (a measure of average blood sugar over roughly three months) down to normal or near-normal levels using the lowest effective dose. It can be used on its own or combined with other diabetes medications, including insulin.
Most people start at 500 mg once daily, taken with their evening meal. From there, a doctor may increase the dose in 500 mg increments, typically waiting about a week between increases, until blood sugar reaches the target range. The maximum dose is usually 2,000 mg per day when used alone, or 2,500 mg per day when combined with insulin.
How Metformin Lowers Blood Sugar
Your liver constantly produces glucose and releases it into your bloodstream, even when you haven’t eaten. In type 2 diabetes, this process runs too aggressively. Metformin’s primary effect is dialing back that overproduction. It does this by reducing energy availability inside liver cells. With less cellular fuel to power glucose manufacturing, the liver simply makes less sugar.
This mechanism is distinct from insulin. Metformin doesn’t force your pancreas to produce more insulin or directly push sugar into cells. Instead, it also makes your body’s existing insulin work more effectively, a property called insulin sensitization. Because it doesn’t spike insulin levels, metformin carries very little risk of causing dangerously low blood sugar on its own.
Prediabetes Prevention
Metformin is sometimes prescribed to people with prediabetes to delay or prevent the progression to full type 2 diabetes. The American Diabetes Association’s 2025 guidelines suggest considering metformin for people between ages 25 and 59 who have a BMI over 35, a fasting blood sugar above 110 mg/dL, or an A1c above 6%. Women who had gestational diabetes during pregnancy are also candidates. In the large Diabetes Prevention Program trial, metformin reduced the risk of developing type 2 diabetes by about 31% compared to placebo over roughly three years.
Polycystic Ovary Syndrome (PCOS)
Although not FDA-approved for this purpose, metformin is widely prescribed for polycystic ovary syndrome, a hormonal condition affecting roughly 1 in 10 women of reproductive age. PCOS is closely tied to insulin resistance, which drives many of its symptoms: irregular periods, excess androgen (male-type hormones), weight gain concentrated around the waist, and difficulty getting pregnant.
Metformin addresses the insulin resistance at the root of these problems. By lowering insulin and blood sugar levels, it helps reduce androgen levels, regulate menstrual cycles, and improve ovulation. For women with PCOS trying to conceive, metformin can restore ovulatory function. It also helps control weight and waist circumference, and reduces the long-term risks of cardiovascular disease and diabetes that come with the condition.
Weight Loss Effects
Metformin is not a weight loss drug, but it does produce modest weight reduction in many people who take it. In the Diabetes Prevention Program trial, participants on metformin lost an average of 2.1 kg (about 4.6 pounds) over the first three years, compared to almost no change in the placebo group. About 30% of people on metformin lost more than 5% of their body weight in the first year, and a follow-up analysis found those individuals maintained an average 6.2% weight loss after 15 years.
The effect is more pronounced in people with obesity. A meta-analysis of 21 trials found metformin reduced BMI by about 1.3 units on average in people with obesity. Another study in people taking antipsychotic medications, which commonly cause significant weight gain, found metformin helped them lose an average of 3.3 kg (about 7.3 pounds) compared to placebo. These numbers are meaningful but modest. Lifestyle changes like diet and exercise consistently outperform metformin alone for weight loss.
Why the Extended-Release Version
The extended-release formulation exists primarily because standard (immediate-release) metformin commonly causes gastrointestinal side effects: nausea, diarrhea, stomach cramps, and bloating. By releasing the medication gradually over several hours, the ER version significantly reduces these digestive issues. This matters because GI side effects are the number one reason people stop taking metformin. The ER tablet is also more convenient, typically taken once daily with dinner rather than two or three times throughout the day.
Kidney Function and Safety
Metformin is processed through the kidneys, so kidney function determines whether it’s safe to take. Doctors use a blood test called eGFR (estimated glomerular filtration rate) to measure how well your kidneys are filtering. The current guidelines are straightforward: metformin should not be used if eGFR falls below 30. Starting metformin is not recommended when eGFR is between 30 and 45. If someone is already on metformin and their kidney function drops below 45, their doctor will reassess whether the benefits still outweigh the risks. The concern is that impaired kidneys can’t clear metformin efficiently, allowing it to build up and, in rare cases, cause a serious condition called lactic acidosis.
Vitamin B12 and Long-Term Use
One underappreciated effect of long-term metformin use is that it can lower vitamin B12 levels. In the Diabetes Prevention Program Outcomes Study, about 19% of people on metformin had low or borderline-low B12 after five years, compared to roughly 10% on placebo. Each additional year of metformin use increased the odds of B12 deficiency by about 13%. B12 deficiency can cause fatigue, numbness or tingling in the hands and feet, and cognitive changes, symptoms that can be mistaken for diabetic nerve damage.
If you’ve been on metformin for several years, periodic B12 testing is a reasonable precaution. B12 deficiency from metformin use is easily treated with supplements once identified.