Metformin Side Effects: What to Know
Metformin is the most commonly prescribed medication for type 2 diabetes and prediabetes, and if a doctor has just started you on it, knowing what to expect from metformin side effects can make the first few weeks much less unsettling. The good news is that the most common side effects are digestive, dose-related, and usually fade as your body adjusts — and there are a few well-established tricks that help most people get through the adjustment period. This guide covers what’s common, what’s rare but serious, the difference between the two main formulations, the vitamin B12 issue worth knowing about for long-term use, and exactly when a side effect is worth calling your doctor about.
Quick answer: The most common metformin side effects are digestive — diarrhea, nausea, stomach upset, and a metallic taste — and they’re significantly more common with the immediate-release version than the extended-release version. Most digestive side effects ease within a few weeks as the body adjusts, and taking metformin with food helps. Long-term use is linked to a modest drop in vitamin B12 levels, so periodic blood work is worth asking about. The rare but serious risk, lactic acidosis, is extremely uncommon in people with normal kidney function, but metformin is sometimes paused before surgery or imaging that uses injected contrast dye. Seek prompt medical attention for unusual muscle pain, difficulty breathing, or extreme fatigue with stomach pain, which can be signs of lactic acidosis.
Considering an SGLT2 inhibitor as an alternative or add-on? See Farxiga side effects for what to expect.
Taking a GLP-1 medication instead? See Trulicity side effects.
What Is Metformin and Why Is It Prescribed?
Metformin is a first-line medication for type 2 diabetes and is also commonly prescribed for prediabetes and, off-label, for conditions like PCOS. It works primarily by reducing how much glucose the liver releases into the bloodstream and by improving how the body’s cells respond to insulin, which helps lower blood sugar without directly triggering the pancreas to release more insulin the way some other diabetes medications do. Because of this mechanism, metformin on its own carries a very low risk of causing dangerously low blood sugar (hypoglycemia), which is one of the reasons it’s typically the first medication tried for type 2 diabetes.
Common Digestive Side Effects
Digestive side effects are by far the most common complaint with metformin, and the numbers are worth knowing so you’re not caught off guard. With the immediate-release version, diarrhea has been reported in roughly half of people in clinical trials, compared to a much smaller share taking a placebo, and nausea or vomiting affects roughly a quarter of people. The extended-release version tends to be considerably gentler on the digestive system, with diarrhea and nausea affecting a much smaller share of people, which is why doctors often switch a patient to extended-release if immediate-release isn’t tolerated well. A metallic taste in the mouth is also a distinctive, well-documented side effect, affecting a small but real share of people.
Why Metformin Causes Digestive Upset
The exact mechanism isn’t fully understood, but metformin is thought to affect the gut lining and alter the balance of bacteria in the digestive tract, along with slowing the absorption of vitamin B12 (more on that below) and bile acids in the small intestine. This combination of effects on the gut is the leading theory for why digestive symptoms are so common, and it’s also why gradually increasing the dose over several weeks, rather than starting at a full dose immediately, is a standard strategy doctors use to improve tolerability.
Vitamin B12 Deficiency With Long-Term Use
This is one of the more important things to know about taking metformin for years rather than months. Even in short-term studies, a meaningful share of people see their B12 levels drop below normal, and longer-term research following people over more than a decade of metformin use has found an association with vitamin B12 deficiency and, in some cases, anemia, along with an increased rate of nerve-related symptoms like tingling or numbness in people who become deficient. Because B12 deficiency can be easy to miss and its symptoms can overlap with other age-related issues, periodic B12 blood testing is a reasonable thing to ask a doctor about for anyone who’s been on metformin for several years, particularly if new fatigue, tingling, or numbness develops.
Other Common Side Effects
Beyond digestive symptoms, some people notice mild tiredness or a headache in the first weeks of taking metformin, and a smaller share experience dizziness. These tend to be mild and, like the digestive side effects, often improve as the body adjusts to the medication. Weight is typically neutral or slightly reduced with metformin, unlike some other diabetes medications that tend to cause weight gain, which is one of the reasons it’s often a preferred first choice.
Lactic Acidosis: A Rare but Serious Risk
Lactic acidosis is the side effect most doctors warn about, but it’s important to keep it in perspective: large studies estimate the incidence at no more than about 4 cases per 100,000 patient-years of metformin use, making it extremely rare in people who are otherwise good candidates for the medication. The risk rises meaningfully in specific situations: significant kidney impairment (metformin is cleared by the kidneys, so reduced kidney function lets it build up), liver problems, heavy alcohol use, being over 65, conditions that reduce blood oxygen (like severe heart or lung disease), and around surgery or imaging procedures that use injected contrast dye. This is exactly why doctors check kidney function before prescribing metformin and periodically afterward, and why it’s sometimes paused for a day or two around a contrast-dye procedure or major surgery.
Metformin and Contrast Dye / Imaging Procedures
If you’re scheduled for a CT scan, angiogram, or another imaging study that uses injected iodinated contrast dye, tell whoever is ordering the test that you take metformin. Contrast dye can, in a small number of people, temporarily affect kidney function, and reduced kidney function is exactly the situation that raises lactic acidosis risk. Because of this, doctors commonly recommend holding metformin for a short period before and after certain contrast procedures, particularly in people who already have reduced kidney function. This is a routine precaution, not a sign that something is wrong, but it only works if the ordering doctor and radiology team know you’re taking it.
The same logic applies to major surgery under general anesthesia, since surgery can temporarily affect kidney function and blood oxygen levels in ways that raise lactic acidosis risk. If a surgery is planned, mention metformin during pre-operative screening even if it seems obvious it would already be on your chart, since clear, direct communication at each step of care is what actually prevents this rare complication rather than any single safeguard on its own.
Immediate-Release vs. Extended-Release Metformin
The formulation matters a lot for how metformin feels day to day. Immediate-release metformin is typically taken two or three times a day and tends to cause more digestive side effects, since it releases its full dose into the digestive tract relatively quickly. Extended-release metformin is designed to release more gradually over the day, is usually taken once daily, and has meaningfully lower rates of diarrhea and nausea in head-to-head comparisons. It’s not automatically the right choice for everyone — it costs more in some cases and isn’t always covered the same way by insurance — but for anyone struggling with digestive side effects on the immediate-release version, asking about a switch is one of the most effective single changes available, short of stopping the medication altogether.
Tips to Reduce Side Effects
A handful of well-established strategies make a real difference in tolerability. Taking metformin with food, rather than on an empty stomach, substantially reduces digestive upset for most people. Starting at a low dose and increasing gradually over several weeks, rather than jumping straight to the target dose, gives the digestive system time to adjust. If digestive side effects remain a problem on the immediate-release version, ask a doctor about switching to extended-release, which is considerably better tolerated for most people. And staying consistent with the timing and dose, rather than skipping doses and doubling up later, helps avoid the digestive flare-ups that come with restarting.
When to Call Your Doctor
Mild digestive upset in the first few weeks doesn’t usually need a call, but a few situations do. Digestive symptoms that are severe, that don’t improve after several weeks, or that lead to significant weight loss or dehydration are worth reporting, since a dose adjustment or a switch to extended-release may help. New tingling, numbness, or unusual fatigue after months or years on metformin is worth a B12 check. And unusual muscle pain or cramping, trouble breathing, unusual sleepiness, slow or irregular heartbeat, or feeling unusually cold combined with stomach pain and extreme tiredness are signs that warrant emergency care, since these can indicate lactic acidosis, even though it’s rare.
Frequently Asked Questions
Do metformin’s digestive side effects ever go away completely?
For most people, yes — digestive side effects are most intense in the first few weeks and substantially improve as the body adjusts, especially when the dose is increased gradually and taken with food. If they don’t improve after a reasonable adjustment period, switching to extended-release or adjusting the dose with a doctor’s guidance usually helps.
Should I take metformin with or without food?
With food. Taking metformin with a meal significantly reduces digestive side effects compared to taking it on an empty stomach, and this is one of the simplest, most effective ways to improve tolerability.
Is the metallic taste from metformin permanent?
No, the metallic taste some people notice is a known side effect that tends to be most noticeable early in treatment and often fades over time, though it doesn’t affect everyone. If it’s persistent and bothersome, it’s worth mentioning to a doctor, since dose or formulation adjustments can sometimes help.
Can I drink alcohol while taking metformin?
Heavy or binge alcohol use is a specific risk factor for lactic acidosis when combined with metformin, so it’s generally advised to limit alcohol intake and avoid heavy drinking while on this medication. Moderate, occasional alcohol use is a conversation to have directly with a prescribing doctor, since individual health factors matter here.
Does metformin cause weight gain?
No — unlike several other diabetes medications, metformin tends to be weight-neutral or is associated with modest weight loss in many people, which is one of the reasons it’s frequently a first-choice medication for type 2 diabetes and prediabetes.
Metformin side effects are, for the large majority of people, a manageable and often temporary part of starting an effective, well-studied medication. Digestive symptoms respond well to simple strategies like taking it with food and increasing the dose gradually, the B12 issue is easy to monitor with periodic blood work, and the rare serious risk of lactic acidosis is concentrated in specific, identifiable situations that doctors already screen for. Knowing what’s ordinary and what’s worth a call puts you in a much better position to stick with a medication that, for most people, works well long-term.
Related Reading
Metformin is often prescribed alongside lifestyle changes for insulin resistance — see our guide to how to reverse insulin resistance.
Not sure if your symptoms point toward insulin resistance in the first place? Read insulin resistance symptoms: the early warning signs.
If metformin was prescribed alongside dietary changes, our prediabetes diet guide covers what to eat and avoid.
Metformin works by helping lower blood sugar — see the symptoms it’s meant to prevent in signs of high blood sugar you shouldn’t ignore.
Track how your treatment is working over time with our A1C Calculator.
Metformin not doing enough on its own? See how a different diabetes medication compares in our guide to glipizide side effects.
Metformin is often the first medication tried for type 2 diabetes, but when it is not enough on its own, a DPP-4 inhibitor like sitagliptin may be added — see our guide to Januvia side effects for what to expect.
Pioglitazone is another oral diabetes medication often combined with metformin — see pioglitazone side effects for how it compares.
Combining this with a first-line diabetes medication? See Victoza Side Effects.
Combining this with a first-line diabetes medication? See Glyburide Side Effects.
This article is for informational purposes only and is not medical advice. It is not a substitute for guidance from a qualified healthcare provider, who can consider your full medical history, kidney function, and current medications. If you experience unusual muscle pain, difficulty breathing, or extreme fatigue with stomach pain while taking metformin, seek emergency medical care immediately. Sources: Drugs.com clinical side effect data for metformin.