Metformin Side Effects: Symptoms to Bring to Your Doctor

Most metformin side effects are stomach-related: diarrhea, nausea, upset stomach, stomach pain, and a metallic taste. The patient leaflet on DailyMed metformin patient leaflet says these often go away after you have taken the medicine for a while, but to tell your prescriber if they bother you a lot, last more than a few weeks, come back, or start later in treatment. Signs of lactic acidosis, a rare but serious problem, are the symptoms to raise right away.

What are the most common metformin side effects?

The DailyMed leaflet lists diarrhea, nausea, and upset stomach as common, and says about 3 in 100 people notice a metallic taste when they start. Cleveland Clinic’s metformin drug page lists diarrhea, gas, headache, metallic taste, and nausea as side effects that usually do not need medical attention unless they continue or bother you.

How often do side effects happen?

The sources give different numbers. Everyday Health says research has found gastrointestinal side effects in up to 75 percent of people who take metformin. Whitney Rehab’s article puts digestive problems at up to 25 percent of people taking it. Because the two estimates differ, ask your prescriber what is typical for someone in your situation. For how clinical studies record adverse events, see Clinical Study Safety Data (our research library).

Which symptoms should prompt a call to your clinician?

Some symptoms are urgent and should be reported right away. Others are worth raising at your next contact with your prescriber.

The leaflet says lactic acidosis is rare, reported in about one in 33,000 patients taking metformin tablets over a year, and it has mostly occurred in people whose kidneys were not working normally. It can be fatal in up to half the people who develop it. For low blood sugar, Cleveland Clinic says to carry a quick sugar such as glucose tablets or hard sugar candy.

As of October 15, 2026, the DailyMed leaflet is a version revised in October 2012 by a repackager, so current labeling may differ. Ask your pharmacist for the current prescribing information. The leaflet also says side effects can be reported to the FDA at 1-800-FDA-1088.

How do formulation and routine affect tolerability?

Metformin comes as immediate-release and extended-release tablets. The leaflet says both have the same active ingredient, but extended-release works longer in the body. Banner Health says its team sometimes changes people to extended-release to help reduce side effects. Everyday Health cites a 2021 systematic review (Tan et al., Frontiers in Pharmacology) that found people on long-acting metformin were less likely to have adverse gastrointestinal events and more likely to stay on the drug. Whitney Rehab says the extended-release form’s side effects are usually milder. The leaflet also includes handling rules for extended-release tablets, so ask your pharmacist for them.

Routine matters too. Banner Health’s endocrinologist says taking metformin with meals can help reduce side effects. Timing and dose are set by your prescriber and pharmacist, so bring any trouble with them rather than changing anything yourself. Keeping a simple log of when symptoms happen, next to your meals, gives your team the information it needs.

Is metformin safe during pregnancy?

The leaflet says to tell your doctor if you are pregnant or plan to become pregnant, because metformin may not be right for you, and to talk through your choices. Cleveland Clinic adds that metformin may cause you to ovulate, so talk with your care team about contraception while you take it, and contact them if you think you might be pregnant. Pregnancy outcomes are not something this page can weigh for you, so that decision belongs with your prescriber.

Is metformin hard on your kidneys or liver?

The sources do not name one organ that metformin is hard on. The kidneys and liver come up because of lactic acidosis. The leaflet says the rare problem has mostly occurred in people whose kidneys were not working normally, that most people with kidney problems should not take metformin, and that the liver needs to work normally because it helps remove lactic acid. It also states: “There is no evidence that metformin causes harm to the liver or kidneys.” Banner Health calls the idea that metformin damages the kidneys a misconception, while noting it is not recommended in severe kidney impairment. A health page from ManipalCigna lists kidney damage among serious side effects, so the sources do not agree. Ask your prescriber how your kidney and liver tests are planned.

Can metformin cause headaches, and what about long-term use?

Headache appears in Cleveland Clinic’s list of side effects that usually do not need medical attention and in Banner Health’s list of common effects. If a headache comes with any symptom from the urgent list above, treat it as urgent. Banner Health says metformin is widely considered safe for long-term use when a provider monitors it. Cleveland Clinic says your care team will monitor your A1C, and lists low vitamin B12 among side effects to report. It advises you to discuss the foods you eat and the vitamins you take with your team. Whitney Rehab advises checking your B12 level regularly.

What should you not do while taking metformin?

• Do not drink a lot of alcohol, whether in binges or regularly. The leaflet says alcohol can increase the chance of lactic acidosis.
• Do not start or stop it, or change how you take it, on your own. Tell your prescriber before surgery or an x-ray with dye. The leaflet says you may need to stop for a short time, and your team will say when to restart.
• Do not share it with anyone, and do not use it for a condition it was not prescribed for.
• Do not start other medicines or supplements without telling your team. The leaflet notes that other medicines can change how well metformin works and that some can raise blood sugar. Cleveland Clinic’s list includes several antibiotics, blood pressure medicines, and diuretics.
• Do not wait out vomiting, diarrhea, fever, or heavy sweating. Cleveland Clinic’s metformin drug page says to talk to your care team right away for these symptoms, because losing too much body fluid may make it dangerous for you to take this medication. The DailyMed metformin patient leaflet says to tell your doctor about severe vomiting, diarrhea, fever, or low fluid intake, and that you may need to stop metformin for a short time.

What happens if you stop taking metformin?

Do not stop on your own. That decision belongs with your prescriber. The leaflet explains that the main goal of diabetes treatment is lowering blood sugar to a normal level, and that high blood sugar can lead to kidney damage, amputations, and blindness. Banner Health says some people take metformin long term while others have their treatment adjusted over time. What happens to your blood sugar after stopping is a question for your prescriber, so ask what your plan would be and what to watch for.

What is a safer alternative to metformin, and can I use natural options instead?

None of these sources names a single alternative as safer, and whether one suits you is a prescriber decision. Banner Health quotes an endocrinologist saying that if someone cannot tolerate metformin, GLP-1 receptor agonists and SGLT2 inhibitors are good choices. Everyday Health says both classes produce results similar to metformin and are considered good treatment options, and that they may also reduce the risk of cardiovascular disease. It notes they can cause stomach side effects similar to metformin and can be expensive. No natural product is named as a replacement for metformin. The leaflet says to keep up exercise and the diet advised for your diabetes while you take the medicine.

What should you bring to your next appointment?

• Which symptoms from the lists above have you had, when they started, and how long they have lasted?
• Would a different formulation make sense for the stomach symptoms you have?
• When are your kidney and liver tests due?
• Do you need a vitamin B12 check?
• If you could become pregnant, what should you know about metformin and contraception?
• What should you do before a scan with dye or a surgery?
• If side effects make you want to stop, what should you do first?

Try this line: “I’ve had diarrhea since I started metformin, and it has lasted more than three weeks. What should we check, and is a different formulation worth discussing?”

For a structure for the visit, read Preparing a Shared Decision Question for Primary Care, then take your log with you using Bringing Home Readings to a Visit.

Your next step: write down each symptom, its date, and whether it is getting better or worse, then call your prescriber or book a visit with that list. This page is general information and does not replace advice from your prescriber or pharmacist.

By the Primary Care Visit Guide Team