Metformin Side Effects and Simple Ways to Manage

Last updated November 10, 2026

The most common metformin side effects are stomach problems, such as diarrhea, nausea, gas and an upset stomach. They often ease as your body adjusts, especially when you take metformin with meals.

Metformin can also lower vitamin B12 over time. A rare but serious risk called lactic acidosis needs emergency care.

This guide explains the side effects and simple ways to manage them. It also lists warning signs that need a doctor right away.

Key Takeaways

• Metformin treats type 2 diabetes and is one of the most widely used diabetes medicines.
• Stomach side effects are the most common and often improve with time.
• Taking metformin with meals, as directed, can reduce stomach upset.
• Long-term use can lower vitamin B12, so ask about testing.
• Lactic acidosis is rare but serious and needs emergency care.

Jump to a section

• What metformin does
• Common side effects
• Ways to manage side effects
• Metformin and vitamin B12
• Lactic acidosis warning
• Extended-release tips
• Alcohol, surgery and scans
• When to get help
• Frequently asked questions

What Metformin Does

Metformin treats type 2 diabetes. MedlinePlus explains that it lowers blood sugar in three ways. It reduces sugar absorbed from food, cuts sugar made by the liver and helps your body respond better to insulin.

It does not cure diabetes. MedlinePlus says to keep taking it even when you feel well. It is not used for type 1 diabetes.

Metformin comes as a regular tablet, an extended-release tablet and a liquid, according to MedlinePlus. Many people take a lower-cost generic version.

Curious about generics? Our guide on generic vs brand-name medicines explains why they work the same way.

Common Metformin Side Effects

MedlinePlus lists these common side effects. Tell your doctor if any are severe or do not go away.

Other listed effects include skin flushing, nail changes, muscle pain and rash. Many stomach effects show up in the first weeks. They often fade as your body gets used to the medicine.

Simple Ways to Manage Stomach Side Effects

Small changes often ease stomach trouble. Always check with your doctor or pharmacist before changing how you take your medicine.

1. Take metformin with meals, as MedlinePlus advises.
2. Eat smaller, balanced meals instead of large heavy ones.
3. Limit very fatty or greasy foods, which can worsen stomach upset.
4. Drink water through the day to replace fluid lost to diarrhea.
5. Ask your doctor whether an extended-release tablet could suit you better.
6. Keep a simple diary of symptoms and meals to share at your next visit.

Doctors often start with a low dose and raise it slowly to reduce stomach effects. Your prescriber decides that schedule, so follow your label exactly.

Metformin and Vitamin B12

The NIH says metformin might reduce vitamin B12 absorption and lower blood levels.

The UK medicines regulator now lists low B12 as a common side effect. It says the risk rises with higher doses, longer treatment and other risk factors.

Signs of Low B12

• Extreme tiredness or weakness
• A sore or red tongue, or mouth ulcers
• Pins and needles, numbness or tingling in hands and feet
• Pale or yellowish skin
• Low mood, memory problems or confusion
• Balance problems

Who Has Higher Risk

• Older adults
• People with Crohn’s disease or past stomach surgery
• People on strict vegan diets
• People who also take acid reducers called proton pump inhibitors

Ask your doctor whether you need a B12 blood test. The MHRA advises against stopping metformin on your own, since doctors can treat low B12 while you continue.

The Lactic Acidosis Warning

MedlinePlus says metformin may rarely cause lactic acidosis, a serious and life-threatening buildup of acid in the blood. It is uncommon, but you should know the signs.

MedlinePlus says to stop metformin and call a doctor right away if you notice these signs.

• Extreme tiredness, weakness or muscle pain
• Nausea, vomiting or stomach pain
• Fast or hard breathing
• Dizziness or an abnormal heartbeat
• Feeling cold, especially in the hands or feet

Extended-Release Tablet Tips

Extended-release metformin releases the medicine slowly through the day. MedlinePlus says to swallow these tablets whole, without splitting, chewing or crushing.

You may see an empty tablet shell in your stool. MedlinePlus says this is normal and does not mean the medicine failed.

Alcohol, Surgery and Scans

Regular or heavy drinking raises the risk of lactic acidosis and can lower blood sugar. Ask your doctor how much alcohol, if any, is safe for you.

Tell your doctor before any surgery or imaging test that uses contrast dye. MedlinePlus says you may need to stop metformin beforehand and wait at least 48 hours before restarting.

Some foods and drinks affect certain medicines. Our guide to grapefruit and medicine interactions explains how this works.

Living Well on Metformin

• Check your blood sugar at home as your care team suggests.
• Keep up with kidney and B12 blood tests.
• Eat regular meals with fiber, protein and vegetables.
• Stay active most days, as your doctor allows.
• Store tablets in a cool, dry place away from children.

Proper storage keeps medicine working well. See our guide on how to store medicines at home.

Some people with diabetes also look into supplements. Our magnesium guide explains what the research shows, but ask your doctor first.

Questions to Ask Your Doctor

• Should I take the regular or extended-release form?
• How often should I check my blood sugar at home?
• Do I need a vitamin B12 test, and how often?
• How often will you check my kidney function?
• Is it safe for me to drink alcohol?
• What should I do if I get sick and cannot eat?

Write your questions down before the visit, since appointments can feel rushed. Ask the nurse or pharmacist if you forget one.

Bring a list of every medicine and supplement you take. Your pharmacist can also check for interactions in a few minutes.

Metformin Myths and Facts

When to Get Help

Call your doctor soon if side effects are severe, do not improve or you notice signs of low vitamin B12.

Get emergency care right away for any of these.

• Signs of lactic acidosis, such as fast breathing, severe weakness, muscle pain or feeling very cold
• Chest pain
• Severe vomiting or diarrhea that stops you from keeping fluids down
• Confusion, fainting or a very slow or irregular heartbeat

If you think someone took too much metformin, call Poison Help at 1-800-222-1222 in the United States, or call 911.

Your Metformin Checklist

• Take metformin with meals as directed
• Follow your prescription label exactly
• Track stomach symptoms for your doctor
• Ask about vitamin B12 testing
• Tell doctors before surgery or dye scans
• Know the signs of lactic acidosis

Test Yourself

Frequently Asked Questions

Do metformin side effects go away?

Stomach side effects often improve over the first few weeks. Tell your doctor if they continue.

Why does metformin cause diarrhea?

Metformin acts in the gut, which can upset digestion. Taking it with meals often helps.

Does metformin lower vitamin B12?

It can. The NIH says metformin might reduce B12 absorption, so ask about testing.

Is it normal to see a tablet in my stool?

With extended-release tablets, yes. MedlinePlus says an empty shell can pass in stool.

Conclusion

Metformin side effects are usually mild stomach problems that ease with time and meals. Watch for low vitamin B12, and know the rare warning signs of lactic acidosis.

Talk to your doctor or pharmacist about any side effect, and never change your dose on your own at home.

Editor’s Note

This guide is accurate as of November 10, 2026. Medicine details come from MedlinePlus. Vitamin B12 guidance comes from the NIH and the UK Medicines and Healthcare products Regulatory Agency.

This is general information, not medical advice. Talk to a health professional about your own treatment.

Spot an error in this article? Please contact us at support@digieh.com or read our Corrections Policy.

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