What Are The Side Effects Of Metformin?

Type 2 diabetes can be managed with diet and lifestyle, but many people also take medications to help manage high blood sugar levels. 

There are several medications available to treat type 2 diabetes today, and one of those medications is metformin. 

As with any medication, there are some potential side effects from taking metformin, which we discuss in this article.

What is metformin?

Metformin is one of the most commonly prescribed medications for type 2 diabetes mellitus. Metformin is the generic name most commonly prescribed, and the brand name for metformin is Glucophage.

This diabetes medication doesn’t cause low blood sugar (hypoglycemia), so this is one of its major benefits. Because it doesn’t cause low blood sugar levels, people with type 2 diabetes can take metformin along with other medications like sulfonylureas and injectable insulin.

Metformin is in a drug class called biguanides. Biguanides help reduce the amount of sugar your liver makes while improving insulin sensitivity, which helps reduce high blood sugar. There are two types of metformin – regular metformin and extended-release metformin (XR).

The common dosage for metformin ranges from 500 milligrams to a maximum of 2,550 milligrams per day. It’s usually recommended to split the dose among meals, typically three times per day. 

While metformin is most commonly used for type 2 diabetes, it’s also used to help treat polycystic ovarian syndrome (PCOS). Polycystic ovary syndrome is a hormonal disorder in women often stemming from insulin resistance. 

PCOS is one of the leading causes of infertility in women. Some healthcare providers prescribe metformin for women with PCOS struggling with infertility.

One of the benefits of metformin is that it may support weight loss and doesn’t promote weight gain like some diabetes medications. If you’re overweight then weight loss of 5-10% of your body weight can improve your blood glucose levels and improve insulin sensitivity.

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Common metformin side effects

The most common metformin side effects are gastrointestinal-related. If you start taking metformin, you may experience some of these side effects:

  • Stomach pain

  • Nausea or vomiting

  • Bloating

  • Gas

  • Constipation

  • Headache

  • Metallic taste in your mouth

Metformin might also reduce your body’s ability to absorb vitamin B12 in your stomach. Approximately 6-30% of people taking metformin experience vitamin B12 deficiency. According to studies, the higher the metformin dose, the greater the risk of vitamin B12 deficiency. 

Vitamin B12 is important for creating DNA and red blood cells. Vitamin B12 deficiency can lead to anemia, which is when you have lower levels of red blood cells than is healthy. 

Anemia from vitamin B12 deficiency is pernicious anemia and comes with symptoms such as:

  • Weakness, tiredness, or lightheadedness

  • Heart palpitations and shortness of breath

  • Pale skin

  • A smooth tongue

  • Constipation, diarrhea, loss of appetite, or gas 

  • Nerve problems like numbness or tingling, muscle weakness, and problems walking

  • Vision loss

  • Mental problems like depression, memory loss, or behavioral changes

We can treat vitamin B12 deficiency with vitamin B12 injections. Oral supplements aren’t usually used to treat pernicious anemia because the issue lies in the stomach’s ability to absorb vitamin B12.

RELATED: What Happens When You Stop Taking Metformin for Diabetes?

Severe side effects/complications

Most people taking metformin experience minor side effects. However, there is a low risk of more severe side effects from taking metformin. 

Lactic acidosis is a serious condition where your body creates more lactic acid than your body can clear. The symptoms of lactic acidosis usually set in quickly and include:

  • Abdominal or stomach discomfort

  • Decreased appetite

  • Diarrhea

  • Fast, shallow breathing

  • Discomfort

  • Muscle pain or cramping

Metformin overdose is the main risk factor for developing lactic acidosis. According to a position statement in the American Diabetes Association’s Diabetes Care, “When metformin is used as labeled, the increased risk of lactic acidosis is either zero or so close to zero that it cannot be factored into ordinary clinical decision making.”

If you have severe renal impairment or kidney disease, you might also be at greater risk of developing lactic acidosis because your kidneys can’t clear the lactic acid from your system. 

Metformin is safe to use when the estimated glomerular filtration rate (GFR) is greater than or equal to 45 milliliters per minute. The GFR is an indicator of how efficiently your kidneys filter blood. A GFR greater than 60 mL/minute is considered normal, and kidney failure occurs when the GFR falls below 15.

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Ways to reduce the side effects of metformin

Everyone responds to metformin differently. Two people taking the same dosages can have completely different levels of tolerance.

Starting with a low dose and gradually increasing it over a few weeks can help reduce the risk of developing unpleasant side effects. For instance, your healthcare provider might have you start with a 500-milligram-tablet once daily and increase it to two tablets after a week if you’re tolerating it well.

Taking metformin with meals is one of the best ways to reduce the risk of side effects. If you take metformin on an empty stomach, then you’re more likely to have gastrointestinal upset. That’s why metformin is meant to be taken with meals.

If you’re struggling with metformin side effects or don’t eat consistently enough to take your metformin two to three times a day, then you might do better with the extended-release version of metformin. 

Glucophage XR tablets are meant to be taken once daily with dinner. The starting dose is 500 milligrams and should then be increased as tolerated based on tolerance and response. The maximum dose of Glucophage XR is 2000 milligrams once at dinnertime.


Metformin is generally a safe and effective medication used to treat type 2 diabetes mellitus and other conditions related to insulin resistance. It’s inexpensive, generally well-tolerated, and doesn’t cause low blood sugar, unlike many type 2 diabetes medications. Side effects will usually decrease once you get used to the diabetes medication.

If your healthcare provider recommends you take metformin, it’s because the expected benefits outweigh any risks, which are quite low for metformin. You can adjust and tailor your medication dosages and regimens to your unique needs, so always reach out to your healthcare provider if you’re having any issues with your medications.

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  1. Lashen H. Role of metformin in the management of polycystic ovary syndrome. Ther Adv Endocrinol Metab. 2010;1(3):117-128. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3475283/
  2. Kim J, Ahn CW, Fang S, Lee HS, Park JS. Association between metformin dose and vitamin B12 deficiency in patients with type 2 diabetes. Medicine (Baltimore). 2019;98(46):e17918. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6867725/
  3. Hur KY, Kim MK, Ko SH, et al. Metformin Treatment for Patients with Diabetes and Chronic Kidney Disease: A Korean Diabetes Association and Korean Society of Nephrology Consensus Statement. Diabetes Metab J. 2020;44(1):3-10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7043977/

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