Key takeaways:
Metformin is FDA approved to treat Type 2 diabetes. It may start to lower blood glucose (sugar) levels within the first week of treatment, but it likely won’t have its full effects for a couple months.
Metformin works by lowering the amount of sugar produced by and absorbed from your body. It also helps your body respond better to its natural insulin. Two signs that metformin is working are that you’ve seen a reduction in your blood sugar and hemoglobin A1C (your average blood sugar over the past 2 to 3 months).
Nausea and diarrhea are common side effects of metformin, but they usually improve within a few weeks of starting the medication. Metformin can also cause rare but serious side effects, such as low vitamin B12 levels and lactic acidosis.
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Metformin is an oral medication that’s FDA approved to treat Type 2 diabetes. It comes in immediate-release (IR) and extended-release (ER) formulations. Metformin IR is approved in adults and children who are at least 10 years old. Metformin ER is only approved for adults.
Metformin is often a first-choice medication to help manage Type 2 diabetes. If you’ve been prescribed metformin, it’s common to have questions about it. From how long it takes to work to signs metformin is working, we’ve got you covered. Below, you’ll find answers to seven frequently asked questions.
1. How long does it take metformin to work?
Metformin may begin working within the first week of starting it. In one study, metformin lowered blood glucose (sugar) within the first week of treatment. But it took about 2 months for it to have its full sugar-lowering effect.
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In addition to lowering blood sugar, metformin also lowers your hemoglobin A1C (HbA1C or A1C). Your A1C is a lab value that represents your average blood sugar levels over the past 2 to 3 months. Metformin can lower A1C by up to 1.5%.
2. How does metformin work?
Metformin works in three main ways to help lower blood sugar:
It reduces the amount of sugar your liver makes.
It reduces the amount of sugar your body absorbs from food and drinks.
It helps your body use its natural insulin more effectively, which can help combat insulin resistance.
In certain cases, healthcare professionals may prescribe metformin off-label to help with weight loss or treat other health conditions, including polycystic ovary syndrome (PCOS) and prediabetes. But keep in mind that metformin isn’t approved for these uses.
3. What are some signs metformin is working?
The most obvious sign that metformin is working is when you see a reduction in your blood sugar and A1C levels. Your endocrinologist (diabetes specialist) should check your A1C before you start metformin, and then periodically throughout treatment (usually every 3 to 6 months). They should also help you identify your target blood sugar and A1C ranges.
Your endocrinologist should tell you how often you should check your sugar levels. Keeping a log (paper or digital) of your levels can help you see sugar trends. As mentioned above, you may start to see some improvements in as little as 1 week of starting metformin. But don’t fret if this isn’t your experience. It can take a few months to notice consistent blood sugar reduction.
4. How long do you have to take metformin?
For many people, metformin is a medication they take long term. Type 2 diabetes is a chronic health condition. And consistent treatment with medications, such as metformin, can help lower the risk of long-term complications, such as kidney or heart disease.
Taking metformin as prescribed helps it to work its best. Don’t stop taking it without your diabetes care team’s guidance. Stopping metformin on your own can cause your blood sugar to go up. And over time, high sugar levels can damage your heart, nerves, eyes, and more.
5. Is metformin safe?
Yes, metformin is safe and well tolerated for most people living with Type 2 diabetes. But side effects can happen. Luckily, many of these are generally mild and go away over time.
Common side effects
Some of the most common side effects of metformin include:
Nausea or vomiting
Gas
Heartburn
Tiredness
Headache
These side effects typically get better or go away as your body adjusts to metformin. Your prescriber should have you start with a lower dosage and raise it slowly. Doing this can help limit side effects. Taking metformin with food can also help.
If your stomach discomfort continues after a few weeks, talk to your endocrinologist. If you’re taking metformin IR, they may recommend switching you to metformin ER.
Serious side effects
Metformin can also cause more serious side effects. These are typically rare but include:
Low vitamin B12 levels: Low vitamin B12 levels can cause or worsen certain health conditions, like anemia (low red blood cell count) or nerve damage. If you’re taking metformin, your endocrinologist will likely recommend routine vitamin B12 testing.
Lactic acidosis: Metformin has a boxed warning (the FDA’s strongest medication warning) for lactic acidosis (a buildup of lactic acid in your blood). The risk of lactic acidosis is higher for people with severe kidney problems and those who drink excessive amounts of alcohol. Get emergency care if you experience symptoms of lactic acidosis, such as extreme fatigue, rapid breathing, or a fast heartbeat.
Hypoglycemia (low blood sugar): On its own, metformin is unlikely to cause hypoglycemia. But it can make hypoglycemia more likely to happen in people who also take other diabetes medications, such as sulfonylureas, glinides, or insulin. Discuss your risk for hypoglycemia and how to treat it if these symptoms occur with your diabetes care team.
6. Is it better to take metformin in the morning or at night?
It depends on the form of metformin you’re taking and your prescribed dosage schedule. Metformin IR is often taken 2 to 3 times daily with meals, such as breakfast and dinner. Metformin ER is usually taken once daily with an evening meal.
Timing metformin with food helps reduce digestion-related side effects, such as nausea. Your healthcare professional will recommend the timing that works best for your treatment plan.
7. Can metformin cause migraines?
Migraines aren’t a common side effect of metformin. However, headaches can occasionally occur when starting the medication or if your blood sugar levels change. In rare cases, long-term metformin use may lower vitamin B12 levels, which can sometimes contribute to headaches for some people.
If you experience frequent or severe migraines while taking metformin, talk with your healthcare professional to help determine the cause.
How to save on metformin
There are many ways to save on metformin, which is available as both a brand-name and generic medication. GoodRx can help make your metformin prescription more affordable. Generic metformin IR’s price at certain pharmacies is as low as $9.00 with a free GoodRx discount.
You can also save on the different forms of metformin ER. Generic metformin ER may cost as little as $11.70. Generic Glumetza may be as low as $64.80. And generic Fortamet's prices are as low as $28.76.
The bottom line
Metformin is a common first-choice medication for managing Type 2 diabetes. It may start lowering your blood glucose (sugar) within the first week of taking it. Consistent effects are usually seen after taking it for 2 months. The biggest sign that metformin is working is when you start noticing lower blood sugar levels.
Metformin is meant to be taken long-term to help treat Type 2 diabetes. And it’s safe for most people to take. Common side effects, such as diarrhea and nausea, are mild and typically get better as your body adjusts to metformin. Metformin can cause more serious side effects, including low vitamin B12 levels and lactic acidosis. But these are generally rare.
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References
A-S Medication Solutions. (2023). Metformin hydrochloride- metformin hydrochloride tablet [package insert].
American Diabetes Association. (n.d.). Health checks for people with diabetes.
American Diabetes Association Professional Practice Committee. (2026). 9. Pharmacologic approaches to glycemic treatment: Standards of care in diabetes—2026. Diabetes Care.
Centers for Disease Control and Prevention. (2024). How diabetes can affect your body.
Corcoran, C., et al. (2023). Metformin. StatPearls.
Diabetes Prevention Program Research Group. (2012). Long-term safety, tolerability, and weight loss associated with metformin in the diabetes prevention program outcomes study. Diabetes Care.
Schwartz, S., et al. (2006). Efficacy, tolerability, and safety of a novel once-daily extended-release metformin in patients with type 2 diabetes. Diabetes Care.
Time Cap Laboratories, Inc. (2025). Metformin HCL tablet, film coated [package insert].
Quallent Pharmaceuticals Health LLC. (2025). Metformin ER 500 MG, 750 MG tablet [package insert]. DailyMed.
