Metformin side effects: how common they are and what helps

Metformin’s label carries a boxed warning for lactic acidosis, a rare but sometimes fatal buildup of lactate in the blood. Its common side effects are milder and mostly in the gut: in the label trial of immediate-release tablets, 53% of people had diarrhea (12% on placebo) and 26% had nausea or vomiting (8% on placebo).

Metformin is an FDA-approved generic for blood sugar control in type 2 diabetes, and millions of people take it. This guide goes through the side effects in the order that matters: the boxed warning first, then the common ones by frequency, the extended-release (ER) difference, vitamin B12, and what people taking it off-label for longevity should know.

The boxed warning: lactic acidosis

The metformin label opens with a boxed warning. Post-marketing cases of metformin-associated lactic acidosis have resulted in death, low body temperature, low blood pressure and slow heart rhythms that resist treatment. The early signs are vague: feeling unwell, muscle aches, trouble breathing, unusual sleepiness and stomach pain.

The label lists the risk factors:

• Kidney impairment
• Certain other drugs, such as topiramate
• Age 65 or older
• Imaging with contrast dye
• Surgery and other procedures
• Low-oxygen states, such as acute heart failure
• Excessive alcohol intake
• Liver impairment
• Mitochondrial diseases

How rare is it? A 2010 Cochrane review pooled 347 trials and cohort studies in type 2 diabetes and found no cases of fatal or nonfatal lactic acidosis in 70,490 patient-years of metformin use or in 55,451 patient-years on other diabetes drugs. The authors put the upper limit of the true rate at 4.3 cases per 100,000 patient-years on metformin and 5.4 on other drugs. Rare isn’t the same as never, which is why the label’s kidney rules exist.

Stop metformin and call your prescriber right away if you feel very weak or tired, have unusual muscle pain, trouble breathing, unusual sleepiness, unexplained stomach problems, feel cold in your hands or feet, feel dizzy, or have a slow or irregular heartbeat. Those are the warning signs the patient information lists.

Side effects of metformin by frequency

These are the side effects reported in more than 5% of people in the label’s US trial of immediate-release tablets (up to 2,550 mg a day), and more often than on placebo.

Diarrhea led 6% of people on metformin to stop. Less common effects, reported in 1% to 5%, include abnormal stools, low blood sugar, muscle aches, lightheadedness, shortness of breath, nail changes, rash, sweating, a change in taste, chest discomfort, chills, flu-like symptoms, flushing and palpitations.

The large prevention trial tells a similar story. In the Diabetes Prevention Program, which gave 850 mg twice a day to adults with prediabetes, stomach symptoms ran at 77.8 events per 100 person-years on metformin vs 30.7 on placebo. In the 10-year follow-up, those symptoms declined over time.

Metformin extended release: fewer stomach side effects?

Extended-release tablets release the drug slowly and are taken once a day with the evening meal. In the label’s placebo-controlled ER trials (781 people on metformin ER, 195 on placebo):

Those numbers look much lower than the immediate-release table, but the label cautions that rates from trials run under different conditions can’t be directly compared. No trial in the label tests the two forms head to head.

A few practical points from the ER label:

• The starting dose is 500 mg once a day with the evening meal, raised by 500 mg a week to a maximum of 2,000 mg once a day.
• Swallow the tablet whole. Never crush, cut or chew it.
• You may see a soft mass in your stool that looks like the tablet. That is the empty shell, not undigested medicine.

Immediate-release tablets usually start at 500 mg twice a day or 850 mg once a day, with meals, up to 2,550 mg a day in divided doses.

Vitamin B12

Metformin can lower vitamin B12, probably by interfering with how it is absorbed. In 29-week label trials, about 7% of people with normal B12 at the start dropped below normal. The label says the drop may be linked to anemia and appears to reverse quickly when metformin is stopped or B12 is supplemented.

Over longer periods the effect adds up. In the DPP Outcomes Study, low or borderline-low B12 was found in 19.1% of people on metformin vs 9.5% on placebo at 5 years, and each year of metformin use raised the odds of deficiency by 13%. The label says to check blood counts every year and B12 every 2 to 3 years. The American Diabetes Association’s 2026 standards add that checking matters most at doses of 1,500 mg a day or more and after 4 to 5 years of use.

Low blood sugar

On its own, metformin rarely causes low blood sugar. The label warns that the risk rises when it’s combined with insulin or drugs that raise insulin, such as sulfonylureas, and that a lower dose of those drugs may be needed. The patient information adds that low blood sugar can happen if you don’t eat enough or drink alcohol.

Metformin side effects in women

The label tables don’t split side effects by sex. Three things are specific to women:

• Tolerance. A Scottish study compared 251 people with type 2 diabetes who couldn’t tolerate metformin with 1,915 who could, and found women and older people were more likely to be intolerant.
• Ovulation. The label warns that metformin may cause ovulation in some premenopausal women who weren’t ovulating, which can lead to an unplanned pregnancy. If pregnancy isn’t the goal, talk with your prescriber about contraception.
• Pregnancy and breastfeeding. The label says published studies haven’t shown a clear link between metformin and major birth defects or miscarriage, but the data are limited. Metformin passes into breast milk, and its effect on the baby isn’t known.

Metformin and alcohol

Alcohol increases metformin’s effect on lactate, so the label tells patients to avoid excessive drinking, whether that means drinking often or binge drinking. Alcohol is listed among the risk factors for lactic acidosis, and the patient information names getting dehydrated as another. The label sets no safe limit, so ask your prescriber what’s reasonable for you.

Rarer reports

Since approval, cases of liver injury (cholestatic, hepatocellular and mixed) have been reported with metformin. Because these reports come from a population of unknown size, the label says their frequency can’t be estimated.

Who shouldn’t take metformin

The label lists three contraindications: an eGFR (a measure of kidney function) below 30, an allergy to metformin, and acute or chronic metabolic acidosis, including diabetic ketoacidosis. Starting metformin isn’t recommended at an eGFR of 30 to 45. Kidney function should be checked before starting and at least yearly after. Metformin should be paused around some contrast imaging and around surgery that limits food and fluids.

What longevity users should know

Several online clinics prescribe metformin to people without diabetes for healthy aging. That use is off-label: the FDA has approved metformin for type 2 diabetes only, and no trial has shown it slows aging or extends life in people without diabetes. Our metformin for longevity guide covers that evidence.

Two side effects matter more for this group:

• The exercise effect. In a 12-week trial of 53 adults averaging 62, metformin blunted the gains in insulin sensitivity and VO2 max from aerobic training. In the MASTERS trial of 94 adults 65 and older, people on placebo gained more lean mass and thigh muscle from 14 weeks of strength training than people on 1,700 mg of metformin a day.
• Years of use. The B12 risk grows with time on the drug, and someone starting at 50 for longevity may take it for decades.

A healthy person taking metformin for longevity accepts the same label risks without a proven benefit, so a kidney test before starting and B12 checks along the way still apply. Our metformin page has the drug facts, and the longevity clinic rankings compare the longevity clinics we list.

What helps

• Take it with food. The patient information says taking metformin with meals helps reduce stomach side effects.
• Start low. The label’s dosing steps up by 500 mg a week for a reason; ask your prescriber about a slower increase if the first weeks are rough.
• Ask about ER. If immediate-release tablets upset your stomach, ask whether extended-release is an option.
• Report what lasts. Tell your prescriber if side effects last more than a few weeks, come back after going away, or start later on. You may need a lower dose or a different drug.

The bottom line

Most metformin side effects are stomach problems that are common early on and tend to fade. The ones worth planning for are rarer: lactic acidosis, the boxed warning, which is why kidney function and alcohol matter, and low B12, which builds with years of use. Extended-release tablets had lower rates of diarrhea in their own trials, though no label trial compares the two directly. If you’re taking metformin off-label for longevity or weight loss, the same rules apply, and the trade-off deserves a direct conversation with your prescriber. For more on the longevity field, see the longevity hub.