Short answer: In the label's placebo-controlled trial, the most common metformin side effects were diarrhea (53% vs 12% on placebo), nausea or vomiting (26% vs 8%), gas (12% vs 6%), weakness, indigestion, abdominal discomfort and headache. Diarrhea led 6% to stop. Metformin has a boxed warning for lactic acidosis, a rare but sometimes fatal build-up of lactic acid in the blood.
What Metformin is approved for: Metformin is a biguanide approved, along with diet and exercise, to improve blood sugar control in adults and children 10 years and older with type 2 diabetes (section 1). The Patient Information says it is not for people with type 1 diabetes or for diabetic ketoacidosis.
Source
Metformin hydrochloride tablets, for oral use, prescribing information and Patient Information. Zydus Pharmaceuticals (USA). DailyMed, version published August 17, 2026. DailyMed setid c82a10fa-1e8e-46b6-890a-737de3f34ee1
WARNING: LACTIC ACIDOSIS
Cases of metformin-associated lactic acidosis reported after approval have resulted in death, low body temperature, low blood pressure and slow heart rhythms that resist treatment. It often starts subtly, with only vague symptoms such as feeling unwell, muscle aches, trouble breathing, sleepiness and abdominal pain. It was marked by blood lactate above 5 mmol/L, anion gap acidosis without ketones, a raised lactate to pyruvate ratio, and metformin levels generally above 5 mcg/mL. Risk factors include kidney impairment, certain drugs (for example carbonic anhydrase inhibitors such as topiramate), age 65 or older, a radiology study with contrast, surgery and other procedures, low-oxygen states such as acute heart failure, heavy alcohol use, liver impairment and mitochondrial diseases. If it is suspected, metformin is stopped immediately and supportive care is started in a hospital; prompt hemodialysis is recommended.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Metformin.
| Adverse reaction | Metformin hydrochloride tablets (n=141) | Placebo (n=145) |
|---|---|---|
| Diarrhea | 53% | 12% |
| Nausea/Vomiting | 26% | 8% |
| Flatulence | 12% | 6% |
| Asthenia | 9% | 6% |
| Indigestion | 7% | 4% |
| Abdominal Discomfort | 6% | 5% |
| Headache | 6% | 5% |
One U.S. trial in type 2 diabetes; 141 people took metformin at up to 2,550 mg per day. Diarrhea led 6% to stop metformin. Asthenia means weakness or lack of energy.
Reported in 1% to 5% of people on metformin and more often than on placebo: abnormal stools, low blood sugar, muscle aches, lightheadedness, shortness of breath, nail disorder, rash, more sweating, taste disorder, chest discomfort, chills, flu syndrome, flushing and palpitations.
In 29-week trials, about 7% of people with previously normal vitamin B12 levels dropped to below-normal levels. The label says this appears to reverse quickly when metformin is stopped or B12 is supplemented.
About 3 out of every 100 people have an unpleasant metallic taste when they start metformin, lasting a short time.
In children with type 2 diabetes, the side effects were similar to those seen in adults.
Reported after approval: cholestatic, hepatocellular and mixed liver injury.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Who should not take it. Metformin is contraindicated with severe kidney impairment (eGFR below 30 mL/min/1.73 m2), a known allergy to metformin, and acute or chronic metabolic acidosis, including diabetic ketoacidosis.
Lactic acidosis. The boxed-warning risk. The label lowers it by checking kidney function (eGFR) before starting and at least once a year, more often in older people; not starting at eGFR 30 to 45; stopping around iodinated contrast scans in higher-risk people; pausing while food and fluids are restricted for surgery; stopping in shock, heart attack, sepsis or acute heart failure with low oxygen; avoiding use with liver disease; warning against heavy alcohol; and not recommending it with mitochondrial diseases such as MELAS and MIDD.
Vitamin B12 deficiency. Metformin may lower vitamin B12, which may be linked to anemia. The label says to check blood counts once a year and vitamin B12 every 2 to 3 years.
Low blood sugar with insulin or sulfonylureas. Metformin may raise the risk of hypoglycemia when used with insulin or an insulin secretagogue such as a sulfonylurea, so a lower dose of those may be needed.
Heart and blood vessel outcomes. No clinical studies have shown conclusive evidence that metformin lowers the risk of large blood vessel disease.
Drug interactions. Carbonic anhydrase inhibitors (topiramate, zonisamide, acetazolamide, dichlorphenamide) and drugs that slow metformin clearance (ranolazine, vandetanib, dolutegravir, cimetidine) may raise lactic acidosis risk. Diuretics, steroids, thyroid products, estrogens and some others can raise blood sugar.
Unplanned pregnancy. Metformin may cause ovulation in some premenopausal women who were not ovulating, which may lead to an unintended pregnancy.
From the Patient Information that comes with Metformin:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.