How Is Metformin Used? Side Effects and Risks

By Zina Semenovskaya, MD
Medically reviewed checkmarkMedically reviewed
September 20, 2022

Metformin is a prescription medication that is frequently used to treat type 2 diabetes mellitus, insulin resistance, and other conditions. It is sold under the brand names Glucophage and Fortamet. Metformin is a first-line treatment for type 2 diabetes, according to the American Diabetes Association. It works by helping to control blood sugar levels and increase the body’s sensitivity and response to insulin.

In this article, we’ll explore common uses and dosages, side effects, risks, and how to know when it is safe to take metformin.

How Does Metformin Lower Blood Sugar?

Metformin works to lower both fasting blood sugar and blood sugar levels after meals. It works by increasing how the body responds to insulin, the hormone that takes sugar out of circulation in the bloodstream and stores it in the cells. 

As insulin removes sugar from the blood, blood glucose levels drop. Type 2 diabetes is characterized by problems responding to insulin, known as insulin resistance. As metformin helps resensitize the body to insulin and its workings, blood sugar levels can become more stable.

Metformin also changes the way that the body absorbs sugar. It decreases how much gets taken into circulation, which keeps levels lower. Metformin also decreases how much glucose is made by the liver, which helps lower the body’s overall circulating glucose levels.

Metformin is able to change the way that the body responds to or processes sugar in these three important ways, making it a highly effective medication for type 2 diabetes.


Metformin is approved by the U.S. Food and Drug Administration (FDA) and prescribed to help control high blood sugar levels for type 2 diabetes.

It may be used for other off-label purposes, including managing glucose levels in people who have prediabetes or gestational diabetes. It may also be prescribed for managing polycystic ovary syndrome (PCOS).

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Metformin comes in both immediate-release and extended-release formulations. The prescribed dosage depends on the patient’s age and the condition being treated. A medical provider will start a patient on metformin at the lowest effective dose.

Common dosages that may be used for metformin include:

  • Oral solution: To start, 5 mL twice per day or 8.5 mL once per day.
  • Oral tablets: To start, 500 mg twice daily, with morning and evening meals, or 850 mg once per day with the morning meal.
  • Extended-release suspension: To start, 5 mL per day. Doses may be gradually increased to a maximum daily intake of 20 mL.
  • Extended-release tablets: To start, 500 to 1,000 mg once per day, taken with the evening meal. Dosage may be increased gradually to 2,000 or 2,500 mg per day.

Side Effects of Metformin

Like most prescription drugs, metformin can cause some common side effects. For most people, the side effects decrease over time as the body adjusts to the medicine.

Common metformin side effects are:

  • Nausea: Because metformin changes how the liver processes and makes glucose, nausea may be a side effect until the body adjusts to the effects of the medicine. Nausea is not usually a long-term side effect but can last for several months.
  • Diarrhea: One of the most common side effects of metformin, diarrhea, and stomach upset can happen because the intestines eliminate more sugar. This can lead to irritation of the intestinal lining. Taking metformin with meals can help to decrease this side effect, but some people continue having diarrhea for as long as they take the medication.
  • Metallic taste in mouth: Around 3% of people who start on metformin may notice a short-term metallic or bad taste in the mouth. It usually resolves after a few days.
  • Weight loss: Because metformin changes the way that the body uses glucose, it can decrease how the body stores excess sugar as fat. This can prevent more fat storage and may lead to weight loss, although it is not a weight loss drug. Not everyone who takes metformin will experience weight loss, and the amount of weight loss can vary.
  • Headache: Another common short-term side effect of Metformin is mild headaches. Let your healthcare provider know if the headaches are severe or last for more than a few days.

This is not a complete list of side effects. You can also develop low blood sugar while taking metformin. Drug interactions between metformin and other prescriptions may also increase the risk for side effects. Let your pharmacist and healthcare provider know of any adverse effects, and make sure they know about all of the other medications, OTC medicine, and herbs or dietary supplements you take.

Managing Side Effects

Metformin is generally well tolerated and causes fewer side effects than other glucose-controlling medicines. As the body adjusts to metformin, many side effects will resolve on their own.

To decrease the chance of side effects, healthcare providers will usually start patients on a low dose and gradually increase it over time as needed. It is also recommended to take metformin with meals, which may minimize side effects. Extended-release metformin may also cause fewer side effects since it is taken less frequently, so your healthcare provider may use that formulation.


While metformin is not known for causing a lot of serious side effects, it does have some risks. The FDA has given metformin a black box warning, the strictest warning label that exists. This is because, in rare cases, metformin can cause lactic acidosis, which can be fatal.

Lactic acidosis happens when too much metformin builds up in the body. Symptoms of lactic acidosis include sudden reddening or flushing of the skin, dizziness, nausea, vomiting, muscle pain, and heart rate changes. If you take metformin and notice any of these symptoms, call 911. Lactic acidosis is a medical emergency.

Metformin comes with other potential risks. It may be unsafe or lead to complications or interactions with the following:

  • Kidney problems or renal function disorders
  • Liver disease or liver problems
  • Heart problems or heart failure
  • Metabolic acidosis

Metformin interacts with radiology iodine contrast that may be used before a CT scan and can interfere with surgical procedures. If you take metformin, your healthcare provider will advise you when to stop taking it before any testing or surgeries.

Can I Take Metformin While Pregnant?

The use of metformin is safe in people who are pregnant or breastfeeding. Metformin can cross the placenta, but research has not found any risks for fetal development or negative pregnancy outcomes

There are greater risks for not controlling blood sugar problems or diabetes during pregnancy, so if your healthcare provider recommends metformin during pregnancy, it is important to take it as directed. Uncontrolled glucose levels or diabetes can cause fetal development problems and preterm labor. In cases where glucose levels are extremely high, it can lead to the tragedy of fetal demise.

If you are trying to get pregnant and taking metformin, research has noted that the medication may even reduce the rate of miscarriage in people with PCOS.

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When to See a Medical Provider

If you take metformin and do not feel that your blood sugar is well-controlled, let your healthcare provider know.

If you have signs of type 2 diabetes, or have a family history of diabetes, stay consistent with regular blood sugar checks and primary care appointments. Type 2 diabetes is easier to manage when it is diagnosed early.

If you take metformin, do not stop taking it without talking to your healthcare provider. If you want to stop taking it, a healthcare professional can help determine a safe way to discontinue the medication and support healthy blood glucose levels in other ways.

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K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Zina Semenovskaya, MD

Dr. Semenovskaya specializes in emergency medicine, and received her medical degree from Weill Cornell Medical College. She is currently the medical director at Remote Emergency Medicine Consulting, LLC and splits her time working clinically as an emergency medicine attending in California and Alaska. She is the first of our doctors to be fluent in Russian.

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