Metformin side effects (common and serious)

Metformin is one of the most prescribed drugs in the world, with over 120 million people taking it for type 2 diabetes and weight loss.

While it is generally well tolerated, like all medications, it can cause side effects.

In this article, we’ll look at how metformin works, its common, rare, and serious side effects, and how to mitigate those side effects.

How does metformin work

Metformin works by reducing the amount of sugar released by the liver during the day and by increasing the body’s sensitivity to insulin.

It also reduces the amount of glucose absorbed from food, which in turn lowers blood sugar levels after eating.

Common side effects of metformin

The most common side effects of metformin include:

  • Diarrhea (mild to severe)
  • Flatulence
  • Nausea or vomiting
  • Flatulence and constipation
  • Generally uncomfortable stomach
  • Heartburn
  • Headache
  • Decreased appetite and weight loss
  • Muscle pain or cramps

Some people also experience an unpleasant metallic taste in their mouth after taking metformin.

Metformin side effects usually come on quickly after the first dose, and while some may improve after a few weeks, others may persist.

That being said, some people may not experience unpleasant side effects until they have been taking the drug for months or a year.

Uncommon (serious) side effects of metformin

Vitamin B12 deficiency

Recent studies have shown that long-term use of metformin can cause vitamin B12 deficiency in some people. If left untreated, vitamin B12 deficiency can cause significant nerve damage leading to the diagnosis of peripheral neuropathy.

If the doctor is not aware of this risk, he may misdiagnose the cause of the patient’s neuropathy as a result of high blood sugar when it could be a side effect of metformin.

This nerve damage is irreversible, but further damage can be prevented by supplementing with a regular dose of vitamin B12.

Anyone taking metformin should have their vitamin B12 levels tested annually. Anyone with insufficient levels should start supplementing with vitamin B12 immediately.

Lactic acidosis

There has been a considerable amount of research into the risk of metformin causing lactic acidosis, a condition where lactic acid builds up in the body and can be fatal.

Metformin comes with a “black box” warning about this risk, the most serious warning issued by the Food and Drug Administration (FDA).

However, most studies have resulted in no cases of lactic acidosis, according to “The Phantom of Lactic Acidosis due to Metformin in Diabetes Patients” in the American Diabetes Association’s (ADA) Diabetes Care journal.

“The number of documented cases of metformin-associated lactic acidosis is small, given how widely metformin is used,” explains the ADA. “The safe use of metformin in patients with contraindications can be seen as evidence that it does not cause lactic acidosis.”

Almost all cases of metformin-related lactic acidosis are due to an overdose, not the normal use of the drug.

“Cases of lactic acidosis due to metformin overdose, particularly in young people without risk factors, suggest that metformin may cause lactic acidosis primarily if given in high doses.”

The patients most at risk of developing lactic acidosis while taking metformin are those with other medical conditions, including those with kidney or liver problems, a history of heart attack or acute heart failure, and those who drink alcohol frequently.

Hypoglycemia (low blood sugar)

Metformin very rarely causes hypoglycemia in people with diabetes NO taking insulin injections as it does not increase insulin production like many other diabetes medicines.

Call your doctor if you have any symptoms of hypoglycemia, which may include:

  • Weakness
  • Tiredness
  • Nausea
  • Vomiting
  • Stomachache
  • Dizziness
  • Vertigo
  • Abnormally fast or slow heartbeat

“False” low blood sugar

While this is not a negative side effect, it is important to keep this in mind when starting treatment with metformin.

When blood sugar levels fall into the “normal” range after being consistently high for some time, “false” symptoms of low blood sugar can occur.

As the body has become accustomed to high blood sugar levels, returning to normal levels can cause dizziness, lightheadedness, nausea and extreme hunger.

This should pass after a few days or weeks as your body adjusts to its new normal blood sugar levels.

If you’re feeling low, always check your blood sugar to make sure you’re not experiencing truly low blood sugar (below 70 mg/dL) that requires glucose treatment.

Allergic reactions to metformin

Allergic reactions to metformin are rare but do happen. Symptoms of an allergic reaction may include:

  • Rash
  • hives
  • Itchy
  • Swelling of the face, lips or tongue
  • Breathing difficulties

If any of these symptoms occur, seek medical attention immediately.

Reducing the side effects of metformin

Fortunately, there are a few things you can do to alleviate, reduce, or completely prevent the side effects of metformin.

Take your dose with a meal, not before

The first is to take your dose of metformin midway through your meal, rather than before you start eating.

Patients report significantly fewer stomach problems if there is already food in the stomach before the metformin dose is digested.

Start with a very low dose

By starting with a very low dose, rather than the full dose your doctor would normally prescribe for your height and weight, you can drastically reduce these initially uncomfortable side effects.

Talk to your doctor about adjusting your doses to give your body time to adjust to the medication.

Read more: Metformin Dosage Guide (minimum and maximum doses)

Request the “extended” version.

Too often, doctors prescribe the regular version without considering the extended release (ER) version.

When taking the ER version, each dose is released slowly over several hours compared to all at once, greatly reducing unwanted gastric side effects. Although it costs more, it can significantly reduce the side effects of metformin.

However, you may need to try the regular version first. This is so your doctor can tell your insurance company that you tried it and it wasn’t the right drug for you. Then they will be more likely to include a more elaborate version of “ER”.

Combine it with other diabetes medications

If you and your medical team are about to start treatment with a GLP-1 drug such as Victoza or Byetta, the side effects of metformin can actually offset the side effects of most GLP-1 drugs, such as constipation.

When taking both drugs as part of a diabetes management plan, the side effects of the two drugs essentially balance out.

Read more: Combination drugs with metformin used in type 2 diabetes

Try something else

There are simply people who cannot tolerate metformin. If you find you can’t stand the side effects, talk to your medical team.

If you decide to stop taking metformin, your medical team can help you find a different type of diabetes medicine to help improve your blood sugar.

Read more: The best alternatives to metformin for treating type 2 diabetes

Who should avoid taking metformin?

As with most drugs, there are several types of patients who should not take metformin.

In the case of metformin, the concern is mainly based on the fact that it may increase the risk of producing too much lactic acid. Clearing the body of lactic acid requires healthy organ function!

If you have been diagnosed with kidney or liver problems: The liver and kidneys play a key role in removing metformin from the body. If your kidneys or liver are already struggling to function, you wouldn’t want to add to their burden by taking metformin.

If you have had a heart attack or acute heart failure: Serious heart disease can affect the amount of blood pumped to the kidneys, which can worsen overall kidney function.

As the kidneys play a key role in removing metformin from the body, patients with a history of heart problems should not take metformin.

If you drink alcohol frequently or in large amounts: Again, because metformin depends on healthy liver and kidney function, a person who regularly drinks large amounts of alcohol would be at increased risk of other problems if they were taking metformin.

Read more: Metformin and alcohol: can you drink while taking metformin?


Although metformin is generally well tolerated, it can cause side effects like any drug.

The most common side effects are mild and gastrointestinal in nature, but more serious side effects such as lactic acidosis and vitamin B12 deficiency can occur.

If you experience any symptoms while taking metformin, talk to your doctor about adjusting your dose, switching to extended-release tablets, or trying a different medication.

If any serious or life-threatening symptoms occur, seek medical attention immediately.

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