Metformin is generally safe, but not everyone should take metformin. Here is what you need to know about the safety of metformin and its connections to diabetes. 

Pill glass with metformin pills

Key facts

  • Metformin is primarily used for managing blood sugar levels in people with prediabetes and type 2 diabetes.
  • This treatment is not suitable for everyone. It’s generally advised against for people with liver disease or severe kidney disease, congestive heart failure, and certain other conditions.
  • Metformin use is also approached with caution in pregnant and breastfeeding women and children. It is not FDA-approved for type 1 diabetes.
  • Metformin is generally considered safe for most people. However, it carries risks like lactic acidosis, especially in those with kidney dysfunction, and can cause vitamin B12 deficiency.
  • Common side effects include gastrointestinal issues (but these often subside over time).

Table of Contents

How does metformin work?

The drug lowers the amount of sugar produced by the liver, decreases sugar absorption in the intestines, and allows individual cells in the body to consume more sugar and use that sugar more efficiently.

Together, these three physiological changes decrease the amount of sugar (in the form of glucose) circulating in the blood.

As a result, the average person taking metformin will see their HbA1c, a measure of glucose control over the previous 2 to 3 months, decrease by about a percentage point, which translates into significantly lower blood sugar.

Who should not take metformin?

It is important to know that not everyone should take metformin. 

If you have kidney issues, your healthcare provider should monitor your kidney function while you’re taking metformin, and people with severe kidney disease or kidney problems who are over age 80 are generally advised not to take the drug.

People with liver disease and congestive heart failure may also be advised not to take the medicine.

Metformin has not undergone rigorous clinical trials in pregnant and breastfeeding women or in children, so use in these populations is generally approached with caution. 

The use of metformin is not FDA-approved for people who have type 1 diabetes. While research indicates that the medicine may help increase insulin sensitivity in people with type 1 diabetes, it has not been established to help improve blood glucose control or meaningfully improve other health outcomes.

Some people still take metformin for type 1 diabetes, often to help with weight loss.

Those who have a known allergy to metformin or to any of its ingredients should not take the medication.

Similarly, people who have experienced a condition called lactic acidosis while taking the drug will generally be advised not to take metformin again. 

Because metformin can also interact with numerous other prescription drugs, it is important to discuss any other medicines you’re taking with your doctor before getting a prescription.

What are the dangers of taking metformin?

It is important to know that metformin is generally considered a safe medication, but as with all drugs, it can have side effects and risks. 

Although rare, one of the most serious risks of taking metformin is a condition called lactic acidosis, which involves a rapid buildup of lactic acid in the blood. 

Clinical research has established that lactic acidosis from metformin occurs in only about 6 people per year out of every 100,000 people taking the drug. However, although rare, the condition can be fatal if not treated promptly. 

Symptoms of lactic acidosis include abdominal pain, muscle cramps, difficulty breathing, and fatigue.

Another potential risk of taking metformin is that it can cause a decrease in vitamin B12 levels.

Prolonged B12 deficiency, which may also occur with anemia, can result in neurological problems, weakness, and fatigue. 

A study of 1,111 patients with type 2 diabetes found the risk of vitamin B12 deficiency was greatest in those who had been taking metformin for more than 6 months and who were taking over 1,500 milligrams (mg) of the drug each day. 

Eating a diet rich in good sources of vitamin B12 — such as animal liver and kidneys, clams, beef, sardines, and fortified cereal — and taking vitamin B12 supplements if recommended by your healthcare provider can help protect against this potential side effect.

There are other, less dangerous side effects of metformin, too. 

Among the most common side effects are:

Most of the gastrointestinal side effects will go away over time and can be eased or avoided completely if a person starts with a low dose and increases it over time. Switching to an extended-release (XR or ER) version of the medication can also help reduce these side effects.

If side effects persist, or if any serious side effects occur, it’s important to discuss them with a doctor or other medical provider.

How long can you be on metformin?

There is no established amount of time for how long a person can take metformin. Metformin is considered a long-term medication for the management of prediabetes and type 2 diabetes. 

However, metformin’s effectiveness in regulating blood sugar levels may slightly decrease over time. As a result, for people who have been on metformin for many years, their medical care team may occasionally review and adjust their dosage as needed to ensure optimal treatment effectiveness.

Metformin therapy is individualized to each person, and the long-term treatment plan depends on the person’s response and the goals of treatment, as well as whether or not they experience any adverse effects from the drug.

Metformin is also useful because it can be used either by itself or in combination with other medications. 

It is important to note that metformin does not cure diabetes but rather is intended to help manage blood sugar levels, which can prevent diabetes complications and help reduce the burden of diabetes. 

When taking metformin, it’s important to monitor blood sugar levels as directed by a doctor or medical professional. This may involve finger-stick glucose tests, wearing a continuous glucose monitor (CGM), and/or getting regular HbA1c blood tests done.

This will ensure that metformin is continuing to work as prescribed and can help inform if dosages need to change over time.

If, in consultation with your healthcare team, you decide to stop taking metformin, the drug will stay in your system for about 4 days.

Is metformin hard on your kidneys? 

Metformin is eliminated from the body through the kidneys and is flushed out through the urine. If the kidneys are not functioning properly, metformin can build up in the bloodstream, increasing the risk of lactic acidosis. 

Because of this, people with severe kidney impairment or kidney disease will generally be directed to avoid taking metformin. Those with milder forms of kidney disease should have their kidney function monitored while taking the medicine if their doctors determine they are candidates for metformin therapy.

People with pre-existing renal disease may have their kidney health further harmed by taking metformin.

However, people who have healthy kidney function do not need to worry. There is no evidence that metformin is hard on healthy kidneys.

Is metformin hard on your heart? 

Metformin is not known to have any negative effects on the heart. While it is important to talk with your doctor if you have a history of heart disease before taking metformin, the drug is generally considered safe for the cardiovascular system.

Current research is not clear regarding whether metformin is protective against heart failure, but it has been established as generally safe for people with a range of cardiac conditions.

However, as always, it is a good idea to discuss all other conditions with your doctor or care team before beginning treatment with metformin.

Safety of metformin for people with diabetes

Overall, metformin is considered to be a very safe medication for people with prediabetes and type 2 diabetes. 

The benefits of the drug for helping manage blood sugar levels and reducing the risk of complications from diabetes generally outweigh the relatively small risks of serious side effects. 

Common side effects, such as abdominal or stomach discomfort, are generally temporary and can be managed or eliminated by starting with small doses and gradually increasing them over time.

You should work closely with your doctor or other medical providers to consistently monitor blood sugar levels, report and discuss side effects, and make adjustments to your treatment plan as necessary. 

Sharing any concerns you have with your medical provider helps the care team to ensure you are receiving the most appropriate care.

By asking questions about side effects and discussing other conditions and medications before beginning metformin therapy, people with diabetes can feel confident in the safety of their treatment plan, including their prescription for daily metformin.

Frequently asked questions

Is it safe to take metformin every day?

Yes. The maximum recommended dose of immediate-release metformin oral tablets is 2,550 mg daily. (Do not take more metformin than you are prescribed.)

If you’re taking more than 2,000 mg of metformin per day, you may benefit from splitting it into 2 to 3 smaller doses throughout the day to minimize the risk of gastrointestinal side effects. However, always ask your prescriber before making any changes to your medication regimen.

At what HbA1c level should you start metformin?

The recommended HbA1c for metformin use varies according to a number of factors. In general, though, metformin use may be recommended when someone’s fasting glucose level is over 100 mg/dL and their 2-hour post-meal blood sugar is 140-199 mg/dL.

If the HbA1c is consistently over 5.7%, metformin may also be recommended.

What are the brand names for metformin?

Some brand names for metformin include Fortamet, Glucophage, Glucophage XR, Glumetza, Riomet, and Riomet ER.