What Are the Side Effects of Metformin?

Metformin’s most common side effects are gastrointestinal: nausea, diarrhea, bloating, and stomach discomfort that typically show up within the first weeks of treatment or after a dose increase. These effects are dose-dependent and usually fade with time, which is why most prescribers start at a low dose and ramp up gradually. But the GI issues, while dominant in conversation, are only part of the picture. Long-term use raises a quieter concern around vitamin B12 levels, and rarer complications like lactic acidosis, though uncommon, have specific risk factors worth knowing about.

The Stomach Problems Come First

If you ask someone who has taken metformin about side effects, the answer is almost always some version of “it messed with my stomach.” Nausea, vomiting, abdominal discomfort, and diarrhea are the classic complaints. They tend to appear soon after starting the drug or stepping up the dose, and for most people they settle down within a few weeks as the body adjusts.¹ Taking metformin with food helps, and using the extended-release formulation instead of the standard version can also reduce GI trouble.²

That said, for a minority of people, the GI problems do not go away. One case report documents chronic diarrhea and weight loss appearing years after treatment began, well past the window most people think of as the adjustment period.¹ This is worth knowing because both patients and clinicians can assume metformin is in the clear once it has been tolerated for a while, when sometimes the drug is the culprit all along.

Why Metformin Upsets the Gut

The mechanism behind metformin’s GI effects is more complex than simple stomach irritation. One significant pathway involves bile acids. Metformin increases the bile acid pool inside the intestine, mainly by reducing bile acid reabsorption in the ileum. That disruption of the normal bile acid recycling loop affects the gut microbiome and changes stool consistency, which likely contributes to the diarrhea many people experience.³ It also affects the way certain gut hormones signal, which ties into metformin’s appetite-reducing effect (more on that below). The bile acid story is one reason why simply “coating the stomach” with food helps only partially. The drug is doing something further down in the intestine, not just irritating the stomach lining.

Extended-Release Versus Standard Tablets

Switching to the extended-release (XR) formulation is one of the most common strategies for people who cannot tolerate standard metformin. The evidence here is mixed in a specific way that is worth understanding. One study found that people who were switched from immediate-release to extended-release saw their rate of any GI side effect drop from about 26% to about 12%, and diarrhea specifically fell from around 18% to 8%.⁴ Another trial comparing the two formulations at equivalent doses found GI side effects roughly twice as common with the standard version.⁵

However, a systematic review and meta-analysis that pooled nine randomized controlled trials found that the extended-release version only meaningfully reduced one specific symptom, dyspepsia, and that the cumulative rates of other key GI symptoms were similar between the two formulations.⁶ The same review noted that the standard version was slightly better at lowering blood sugar, though the difference was small enough to be clinically unimportant. So the switch is worth trying if you are struggling with stomach problems, but it is not a guarantee, and the improvement may be more modest than advertised.

Vitamin B12 Depletion Over Time

This is the side effect that sneaks up on people. Unlike the GI complaints that hit early, vitamin B12 deficiency develops gradually over years of metformin use. A randomized placebo-controlled trial found that metformin treatment led to an average 19% decrease in B12 levels compared to placebo over about four years. The absolute risk of outright B12 deficiency was over 7 percentage points higher in the metformin group.⁷ Data from the Diabetes Prevention Program Outcomes Study showed that each additional year on metformin raised the odds of B12 deficiency by about 13%.⁸

Multiple observational studies and meta-analyses have confirmed this relationship, with higher doses and longer treatment duration both increasing the risk.⁹ The problem is that B12 deficiency often goes unrecognized because its symptoms, which include fatigue, memory problems, mood changes, and numbness or tingling in the hands and feet, overlap heavily with symptoms of diabetes itself or of aging. If your doctor does not specifically test for B12, the deficiency can be missed for years.

When B12 Deficiency Leads to Nerve Damage

The connection between metformin-driven B12 deficiency and neuropathy deserves its own attention because it creates a frustrating clinical puzzle. Diabetes itself commonly causes peripheral neuropathy, the tingling and numbness that starts in the feet and hands. If metformin is simultaneously depleting B12, which also causes neuropathy, the drug meant to manage the disease may be quietly making one of its worst complications worse. Studies have found that metformin users have both lower B12 levels and higher rates of peripheral neuropathy and autonomic neuropathy compared to non-users.¹⁰

A recent study using the All of Us research database confirmed that long-term metformin use is associated with both increased B12 deficiency risk and peripheral neuropathy, and recommended that clinicians routinely check B12 status in long-term users, especially when evaluating neuropathy symptoms.¹¹ Not every study has found the neuropathy link to be statistically significant; one study of Saudi individuals with type 2 diabetes found a strong association between metformin use and B12 deficiency (roughly a fivefold increase in odds) but no significant difference in neuropathy test results between metformin users and non-users.¹² The practical takeaway is straightforward: if you have been on metformin for more than a year or two, periodic B12 testing is a reasonable precaution, and supplementation is cheap and effective if levels are low.

Lactic Acidosis Is Rare but Real

Lactic acidosis is the most feared side effect of metformin, and it was the reason an older related drug, phenformin, was pulled from the market decades ago. Metformin can reduce the body’s ability to clear lactate by interfering with certain energy-production pathways in cells. When lactate accumulates faster than the body can process it, blood becomes dangerously acidic.¹³ In practice, though, metformin-associated lactic acidosis (often abbreviated MALA) is uncommon in people with healthy kidneys. It almost always occurs in the context of something else going wrong, typically impaired kidney function that prevents the body from eliminating metformin normally.

The key risk factors include kidney impairment from any cause, heavy alcohol consumption, severe dehydration, and certain medications that interfere with metformin’s elimination.¹³ If you are a clinician or a patient with diabetes who has significantly reduced kidney function, the concern is legitimate and the drug doses need to be adjusted accordingly. For otherwise healthy metformin users, lactic acidosis is not something to lose sleep over.

Alcohol and Metformin Are a Poor Combination

Moderate, occasional drinking is not typically a problem for metformin users, but heavy or binge drinking deserves extra caution. The mechanism is specific: when the liver metabolizes alcohol, it shifts the body’s chemical balance in a direction that favors lactate production and blocks a key process called gluconeogenesis (where the liver makes glucose). Metformin independently inhibits that same process. When both are happening at once, the combined effect raises the risk of lactic acidosis and dangerously low blood sugar.¹⁴ This is one of the few scenarios where metformin’s side-effect risk is substantially elevated in people with otherwise healthy kidneys.

Kidney Function and Dose Adjustments

Because the kidneys are the primary route for eliminating metformin from the body, reduced kidney function is the single most important variable in determining whether the drug is safe and at what dose. Current guidance no longer considers metformin completely off-limits for people with mild to moderate kidney impairment, as it was for years. Modeling studies have shown that safe and effective doses can be maintained across a wide range of kidney function by adjusting the daily amount: people with normal kidney function can take up to about 2,250 mg per day, while those with more advanced chronic kidney disease (stage 4) should stay at 500 mg daily to keep blood levels within a safe range.¹⁵

In the global DISCOVER study, having chronic kidney disease and being aged 75 or older were both independently associated with higher odds of discontinuing metformin, and adverse effects were cited as a reason for changing therapy about five times more often among those who stopped the drug versus those who continued it.¹⁶ The overall discontinuation rate in that study was about 15%, which gives a sense of how often side effects or other concerns lead people to stop.

Contrast Dye and Metformin

If you have ever been told to stop metformin before a CT scan with contrast dye, you might have wondered what the actual risk is. The traditional concern was that iodinated contrast media could cause a sudden decline in kidney function, which would then impair metformin elimination and trigger lactic acidosis. The evidence, however, has not supported this fear in patients with reasonably healthy kidneys. A consensus statement noted that metformin does not increase the risk of contrast-induced kidney injury, and there have been no documented cases in properly selected patients.¹⁷ A systematic review and meta-analysis confirmed this, finding no evidence that continuing metformin during contrast administration is associated with higher rates of kidney injury, lactic acidosis, or kidney deterioration compared to stopping the drug.¹⁸ Many radiology departments have loosened their protocols accordingly, though some still ask patients with existing kidney issues to hold the drug as a precaution.

Weight Changes and Appetite

Unlike most diabetes medications, metformin tends to promote modest weight loss rather than weight gain. This is generally counted as a benefit, but it is worth understanding the mechanism because it overlaps with the side-effect profile. Research suggests the weight change comes primarily from reduced calorie intake rather than any increase in energy expenditure. Metformin appears to affect appetite regulation both directly and through its gastrointestinal effects. In studies of people with diabetes, it had a dose-dependent effect on how much food was consumed, and one 24-week trial saw weight loss of up to 8 kg.¹⁹

A placebo-controlled trial in overweight children found that metformin significantly reduced hunger ratings and food intake after a test meal, and increased feelings of fullness.²⁰ The question of how much weight loss is attributable to “feeling less hungry” versus “feeling queasy” is difficult to tease apart, and the honest answer is probably both, especially in the early months. For people who are overweight, this effect is generally welcome. For people who are already at a healthy weight or underweight, it is something to monitor.

Metformin on Its Own Rarely Causes Low Blood Sugar

One of metformin’s advantages over many other diabetes drugs is that it does not typically cause hypoglycemia when used alone. The drug works mainly by reducing glucose production in the liver and improving insulin sensitivity, rather than by stimulating the pancreas to release more insulin. This means that on its own, metformin almost never drives blood sugar dangerously low. The risk of hypoglycemia increases when metformin is combined with other medications, particularly sulfonylureas or insulin. A large cohort study found that adding either insulin or a sulfonylurea to metformin was associated with measurable rates of hypoglycemia, on the order of 25 to 31 events per 1,000 person-years.²¹ If you are on metformin alone and experiencing symptoms that feel like low blood sugar, it is worth looking for another explanation.

Drug Interactions That Affect How Metformin Works

Metformin relies on specific transport proteins (called organic cation transporters) to get into liver cells, muscle cells, and other tissues where it does its job. Drugs that block these transporters can interfere with metformin in two ways: they can reduce metformin’s effectiveness by keeping it out of target tissues, and they can raise blood levels of metformin by reducing its elimination, which increases lactic acidosis risk.²²

One clinically relevant interaction involves proton pump inhibitors, the widely used acid-reducing drugs. Lab research has shown that these medications inhibit the transport proteins that metformin uses to enter cells, potentially reducing the drug’s glucose-lowering effect. Since only about 60% of metformin-treated patients respond well to the drug, researchers have suggested that concurrent use of proton pump inhibitors may be an unrecognized contributor to poor metformin response in some people.²³ This has not been confirmed in large clinical trials, but it is an intriguing possibility given how commonly these two drug classes are prescribed together.

Effects on Thyroid Hormones

An effect that gets much less attention is metformin’s influence on thyroid-stimulating hormone (TSH). A meta-analysis found that in people with hypothyroidism (underactive thyroid), metformin treatment was associated with a significant decrease in TSH levels. Free T4, the active thyroid hormone, increased slightly but not significantly.²⁴ In practical terms, this means that if you are taking both metformin and thyroid-replacement medication, your thyroid hormone doses might need adjustment over time. It also means that a TSH level drawn while you are on metformin may read lower than it truly is once the drug is stopped. This effect appears most relevant to people who already have thyroid disease, but it is something clinicians should factor in when interpreting lab results.

Metformin During Pregnancy

Metformin crosses the placenta, which makes its use during pregnancy a subject of ongoing debate. It is increasingly prescribed for gestational diabetes, polycystic ovary syndrome-related fertility treatment, and pre-existing type 2 diabetes in pregnancy. A register-based cohort study found that metformin exposure was not associated with increased risk of major congenital anomalies, neonatal mortality, or hyperglycemia in the infant, but it was associated with a higher likelihood of the baby being small for gestational age.²⁵ A review of the literature noted that most of the current evidence focuses on benefits to the mother, and that few randomized trials have examined long-term outcomes in children exposed to metformin in utero, with those that exist lacking adequate sample sizes and follow-up duration.²⁶ The short version: metformin appears reasonably safe during pregnancy based on the data available, but the data on the child’s long-term health are genuinely thin, and this is an area where the science has not caught up with the prescribing.

Metformin and Exercise Performance in Older Adults

As metformin gains attention as a potential anti-aging therapy, researchers have started asking whether it interferes with the benefits of exercise, particularly in older adults. A randomized controlled trial in healthy older adults found that a clinical dose of metformin did not change maximal mitochondrial respiration rates in skeletal muscle, but it did increase the production of hydrogen peroxide in mitochondria, a marker of oxidative stress, by roughly twofold compared to controls.²⁷ The clinical significance of this is still being worked out, but it raises the possibility that metformin could blunt some of the cellular adaptations that make exercise beneficial for muscle health and aging. For people taking metformin specifically for its potential longevity benefits rather than for diabetes, this is an uncomfortable question mark, because the drug might be working against one of the most reliably beneficial anti-aging interventions there is: regular physical activity.