Metformin side effects and how to avoid them

Metformin is an anti-diabetic medication (technically classified as a biguanide) that doctors commonly prescribe to treat type 2 diabetes and prediabetes. It helps control blood glucose levels and restore the body’s response to insulin. Metformin can also treat polycystic ovary syndrome (PCOS). Popular brand names of metformin include Glucophage, Fortamet, Glumetza, and Riomet.

Both type 2 diabetes and PCOS are common health conditions, each with more than 200,000 new cases in the U.S. every year. Millions of adults are at risk for type 2 diabetes, according to a CDC public health notice. 

If you’ve been diagnosed with either condition, chances are, metformin is a treatment option for you. Learning about metformin side effects, warnings, and interactions is a helpful first step toward gaining a better understanding of the drug.

What is metformin?

Metformin is most commonly prescribed to help people with type 2 diabetes. Type 2 diabetes is caused by insulin resistance or decreased insulin sensitivity, which means that the body doesn’t properly respond to insulin. People with type 2 diabetes or prediabetes have hyperglycemia (high blood sugar). Metformin works by slowing the release of glucose from the liver and by slowing the body’s absorption of glucose, both of which help lower blood sugar levels. It is also believed that metformin increases insulin sensitivity, which helps with lowering glucose levels. 

Metformin can also be prescribed off-label to treat polycystic ovary syndrome (PCOS), a condition that can cause elevated insulin levels that increases risk for diabetes. This medication helps normalize insulin levels and may improve fertility.

Common side effects of metformin

Learning about the potential side effects of metformin is just as important as learning about the benefits. As with any medication, there are always possible risks. These are some of the most common metformin side effects: 

• Diarrhea
• Nausea
• Upset stomach
• Metallic taste in the mouth
• Vomiting
• Flatulence 
• Weight loss
• Loss of appetite
• Heartburn 
• Bloating
• Cough
• Sleepiness
• Constipation
• Headache 
• Painful or difficult urination 
• Asthenia 
• Decreased levels of vitamin B12

Does metformin cause weight loss?

A common concern about metformin is that it causes weight loss. While it can cause weight loss by changing the way the body stores fat, this doesn’t mean that everyone who takes the drug will lose weight, or that you should use it to lose weight.

How long do metformin side effects last?

Side effects may start when you first start taking metformin. For some people, side effects will go away soon after their body adjusts to the medication. For others, side effects may linger or become worse. Most people take metformin for long periods of time, so if side effects don’t go away, they could potentially cause long-term damage. Some metformin side effects may go unnoticed, so regular checkups are essential. 

Serious side effects of metformin

Metformin is associated with some severe, long-term side effects. Some serious side effects caused by metformin are dangerous and require immediate medical attention. If you experience any of the following, consult with a healthcare professional right away: 

• Tiredness
• Unusual sleepiness
• Trouble breathing
• A slow or irregular heart rate
• Lightheadedness or dizziness

Taking metformin for extended amounts of time may result in long-term side effects. Metformin can affect the body’s ability to absorb vitamin B12 and cause a vitamin B12 deficiency. It’s often necessary for people taking metformin to get regular blood tests to check their vitamin B12 levels. 

Although it’s rare, metformin may cause allergic reactions. Signs of an allergic reaction include difficulty breathing, swelling of the face or hands, and a skin rash. You should seek immediate medical attention if you believe you’re experiencing an allergic reaction. 

Lactic acidosis

Taking metformin may cause a serious condition called lactic acidosis, which is a buildup of lactic acid in the bloodstream. Lactic acidosis can cause serious side effects and almost always requires hospitalization. Drinking excessive amounts of alcohol while on metformin can significantly increase the risk of getting lactic acidosis. Signs of lactic acidosis include:

• Dizziness or lightheadedness
• Muscle pain
• Extreme weakness or tiredness
• Trouble breathing 
• A fast or slow heart rate
• Flushing of the skin
• Decreased appetite
• Severe stomach pain

If you’re experiencing serious side effects from taking metformin, it’s best to call your doctor immediately. He or she will give you advice on what to do next, and may tell you to stop taking metformin. 

Metformin warnings

Metformin is not the right medication for people with type 1 diabetes. It’s generally considered okay for children and adults with type 2 diabetes or prediabetes, as long as a doctor approves it, but dosages will vary. For adults over the age of 80, metformin is prescribed with caution on a case-by-case basis.

People with certain health conditions have an increased risk of experiencing side effects from taking metformin. Anyone with congestive heart failure, compromised kidney function, poor liver function, or diabetic ketoacidosis should not take metformin.

People with blood problems, kidney problems, kidney disease, liver disease, or breathing problems should be cautious of taking metformin. Having any of these conditions and taking metformin may cause metformin to be less effective or cause additional health complications, such as lactic acidosis. This condition causes a buildup of lactic acid in the blood and can potentially be fatal.

Some studies suggest that metformin may have risk factors that go beyond common side effects. The Food and Drug Administration (FDA) is investigating whether metformin contains carcinogens, and a 2018 study suggests that metformin may reduce some of the positive benefits of aerobic exercise for older adults.

Meanwhile, some metformin myths have been busted. Some hypothesized that metformin caused dementia but a study published in February 2019 found that “metformin use is associated with a reduced risk of dementia.”

Although metformin doesn’t work for some people, these warnings shouldn’t negate the fact that it still helps many people around the world manage their health conditions.

Metformin interactions

Certain medications may make metformin less effective or worsen side effects.

For example, taking metformin with insulin releasing pills or insulin can cause hypoglycemia (low blood sugar), according to the Diabetes Teaching Center at the University of California. Many people who take metformin will need to monitor their blood levels closely. Signs of hypoglycemia include dizziness, shaking, confusion, fatigue, and fainting.

“Metformin can interact with other medications, including diuretic medications, steroid medications, certain thyroid drugs, oral contraceptives, and calcium channel blocking drugs like nifedipine,” says Chirag Shah, MD, and co-founder of Push Health, an online healthcare platform. “Due to a wide range of medication interactions, it is important to review a patient’s current medications before prescribing metformin.”

Creating a list of all the supplements and prescription drugs you take and sharing it with your doctor will help you avoid side effects that might come from taking metformin with something else.

The following medications may interact negatively with metformin: 

• Quinolone antibiotics
• Thiazide diuretics
• Verapamil 
• Medicines administered before MRIs, X-Rays, or CT scans 
• Ethanol
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Steroid medicines
• Thyroid medicines 

A doctor or other medical professional can give you a complete list of drugs that interact negatively with metformin. 

How to avoid metformin side effects

1. Take consistent dosages

It’s best to follow the manufacturer’s instructions on how to use the drug to reduce your chances of experiencing metformin side effects. Most patients take metformin every 12 hours. Consistently taking the appropriate dose at the right time can help reduce side effects.

The standard dosage of metformin for adults is 1000 mg taken twice daily with meals. Taking metformin with food can help eliminate or reduce stomach-related side effects. Missing or skipping a dose of metformin can make side effects worse.  

2. Make lifestyle and diet changes

Lifestyle changes are another key component of managing type 2 diabetes or prediabetes. Exercising regularly can positively affect blood pressure and blood glucose levels for people with type 2 diabetes. Certain foods like refined sugar, alcohol, and hydrogenated oils can cause high blood sugar, so avoiding them can be very beneficial.

Diet and exercise may not serve as complete alternatives to metformin, but they can still improve quality of life. The best way to reduce your chances of experiencing side effects while taking metformin is to talk to your doctor. 

3. Seek alternatives

There are alternative options to metformin for people with health conditions that prevent them from taking metformin, or for those who can’t take metformin because of its side effects. Those with type 2 diabetes may benefit from SGLT2 inhibitors, GLP1 medications, alpha-glucosidase inhibitors, gliptins, or pioglitazone.

Talking with your doctor is the best way to learn about other medications that may be a good fit for you based on your medical history and individual symptoms.

Metformin vs. metformin ER

Metformin and metformin extended-release (ER) are essentially the same medication, but patients don’t take metformin ER as frequently. ER stands for extended-release, meaning the body absorbs metformin ER more slowly than regular metformin. Both drugs help people with type 2 diabetes and prediabetes control their blood sugar levels.

Metformin, sometimes called metformin immediate-release (IR), is often taken twice a day. Metformin ER is taken less frequently, typically once per day. The standard dosage of metformin ER for adults with type 2 diabetes is 1000–2000 mg. Some people may need to take metformin ER twice daily. A healthcare professional can determine this on a case-by-case basis. 

Metformin ER is the generic version of the brand name Glucophage XR. Patients who have stomach upset with regular metformin may switch to Metformin ER, which is better tolerated. Take metformin ER as instructed by your doctor. Some doctors may recommend taking metformin ER once in the morning with breakfast or in the evening with dinner. Taking metformin ER with food can help reduce the risk of side effects like upset stomach and diarrhea.

Metformin vs. metformin ER side effects

Similarly to metformin, metformin ER side effects may include:

• Nausea
• Diarrhea
• Heartburn
• Headache 
• Metallic taste in the mouth
• Tiredness
• Lightheadedness
• Muscle aches or pains
• Slow or irregular heartbeat 

Like metformin, metformin ER can increase the risk of lactic acidosis. 

A medical professional can give a complete list of side effects. The best way to learn more about metformin and metformin ER side effects is to talk with your healthcare provider.