Can Metformin Cause Fatigue and How Can You Manage It?

Metformin can cause fatigue, and it does so through more than one pathway. The drug’s core mechanism involves slowing down part of the energy-production machinery inside your cells, and over months to years it can also deplete vitamin B12, a nutrient essential for energy and nerve function. For most people the tiredness is mild and manageable, but understanding where it comes from makes it far easier to address.

How Metformin Interferes With Cellular Energy

Your cells generate most of their energy inside structures called mitochondria. Metformin has been shown consistently to inhibit a step in this process known as Complex I, which is the entry point for electrons into the chain of reactions that ultimately produces the fuel your cells run on.¹ In plain terms, metformin puts a gentle brake on one of the engines that power every cell in your body. That brake is what makes the drug useful: by dialing down energy production in liver cells, it reduces the amount of glucose the liver pumps into the bloodstream. But the same brake acts on muscle cells, heart cells, and other tissues too.

Lab studies confirm that metformin inhibits Complex I activity and reduces overall cellular respiration.² Research on isolated mitochondria has shown that the drug’s suppression of energy synthesis traces specifically to this Complex I inhibition rather than to some broader disruption of the energy chain.³ For most people at standard doses, the reduction is modest enough that it doesn’t feel like anything dramatic. But if you already have low fitness, chronic illness, or other factors straining your energy reserves, even a modest reduction in cellular energy output can show up as noticeable fatigue.

Reduced Exercise Capacity

One of the more concrete demonstrations of metformin’s energy-dampening effect comes from exercise testing. A study of patients with metabolic syndrome measured oxygen uptake before and after six weeks on metformin. Oxygen uptake dropped from roughly 1.1 liters per minute to about 0.9 liters per minute, and oxygen uptake per kilogram of body weight fell from about 14 to about 11 ml per kilogram per minute.⁴ Those are substantial reductions. In practical terms, activities like climbing stairs, brisk walking, or moderate housework may feel harder than they did before you started the medication.

This doesn’t mean you should avoid exercise on metformin. Physical activity remains one of the best ways to manage blood sugar and improve insulin sensitivity. But if you’ve noticed that workouts feel more taxing or that you run out of steam sooner than expected, the drug’s effect on oxygen processing is a plausible explanation. Adjusting workout intensity gradually and paying attention to how your body responds day to day can help you find a sustainable level of exertion.

The Vitamin B12 Problem

Fatigue that develops gradually over months or years on metformin often has a different culprit: vitamin B12 deficiency. Evidence accumulated over the past two decades shows that metformin-treated patients with type 2 diabetes are at elevated risk of low B12 levels.⁵ B12 is essential for red blood cell formation and nerve function. When it gets low, you feel tired, foggy, and sometimes weak or tingly in the hands and feet.

The mechanism is well characterized. Metformin disrupts B12 absorption in the part of your gut called the ileum by interfering with a calcium-dependent process that normally shuttles B12 across the intestinal wall.⁶ This is not a dramatic block but a slow, steady reduction in the amount of B12 your body takes in each day. Over months, especially if your dietary B12 intake is already borderline, your stores quietly drop. A pilot study using a tracer to track B12 absorption found that metformin cut B12 bioavailability from about 43% to about 31%, and that adding calcium brought it back up to about 46%.⁷

That finding points to a straightforward fix: taking a calcium supplement alongside metformin can reverse the B12 absorption issue. Additionally, the American Diabetes Association recommends periodic B12 screening for patients on long-term metformin therapy, though surveys suggest many physicians are not consistently following those guidelines.⁸ If your doctor hasn’t checked your B12 levels recently and you’ve been on metformin for more than a year, it’s worth asking.

Hypoglycemia and Daytime Exhaustion

Metformin is often described as having a low risk of causing hypoglycemia compared with drugs like sulfonylureas or insulin. That’s generally true, but it doesn’t mean it never happens. A case report documented a patient who experienced chronic daytime fatigue and somnolence so severe he relied on one to two pots of coffee daily just to stay functional. Continuous glucose monitoring revealed he was spending up to 4% of the time in a hypoglycemic range. Within two weeks of discontinuing metformin, the “attacks” stopped and his chronic fatigue improved dramatically.⁹

Cases like this are unusual, but they matter because the fatigue pattern can be misleading. When blood sugar dips briefly and repeatedly, it doesn’t always cause the classic symptoms of hypoglycemia like shaking or sweating. Instead, you may just feel persistently drained, foggy, or sleepy, symptoms that are easy to blame on poor sleep, stress, or aging. If your fatigue has an episodic quality, worsens between meals, or improves right after eating, it’s worth discussing the possibility of subtle hypoglycemia with your doctor. Continuous glucose monitoring or even tracking blood sugar with a standard meter around periods of tiredness can help sort this out.

Gastrointestinal Side Effects and Electrolyte Loss

Metformin’s most common side effects are gastrointestinal: nausea, bloating, cramping, and diarrhea. For most people these are mild and fade after a few weeks. But in some cases, chronic diarrhea can become severe enough to cause meaningful electrolyte disturbances. A case report described a patient who developed symptomatic drops in potassium, magnesium, calcium, and phosphorus from metformin-induced diarrhea, ultimately requiring hospitalization, despite multiple outpatient visits for the diarrhea beforehand.¹⁰

You don’t have to reach that extreme for the issue to matter. Even moderate, ongoing diarrhea depletes fluids and minerals your muscles and nervous system depend on. Low magnesium alone can cause fatigue, muscle cramps, and brain fog. If you’ve been tolerating persistent loose stools and chalking it up as “just a metformin thing,” it may be worth checking whether the GI symptoms are quietly draining your electrolytes. Blood tests for magnesium, potassium, and calcium are inexpensive and widely available.

When Fatigue Signals Something More Serious

Rarely, metformin-related fatigue can be an early sign of lactic acidosis, a dangerous buildup of lactic acid in the blood. Symptoms often start with weakness, nausea, abdominal pain, and shortness of breath before progressing to more severe illness.¹¹ Case reports describe near-fatal episodes of severe metformin-induced lactic acidosis, though these are overwhelmingly concentrated in patients with additional risk factors.¹²

The biggest risk factor is impaired kidney function. Your kidneys are responsible for clearing metformin from the body, and when they aren’t working well, the drug accumulates to levels that can push lactic acid production past the danger threshold.¹³ Dehydration, heavy alcohol use, and acute illness can also tip the balance. If you experience sudden-onset fatigue accompanied by nausea, rapid breathing, or muscle pain, particularly if you’ve been vomiting, ill, or haven’t been eating or drinking normally, seek medical attention promptly. Lactic acidosis is rare but treatable when caught early.

Metformin and Thyroid Function

Thyroid disorders are common in people with diabetes, and an underactive thyroid is itself a major cause of fatigue. Some research has explored whether metformin directly affects thyroid hormone levels. A randomized, placebo-controlled trial in people with prediabetes found that metformin lowered thyroid-stimulating hormone (TSH) levels, but only in those whose TSH was in the high-normal range (above 2.5 μU/ml) at baseline. In people with TSH already in the low-normal range, there was no significant change.¹⁴

This is actually reassuring for most people. If your thyroid is functioning normally, metformin is unlikely to push it into a state that causes fatigue. However, if you already have borderline thyroid function or are on thyroid medication, the interaction is worth monitoring. The practical point: don’t assume all your fatigue is from metformin without checking thyroid levels, especially since the two conditions overlap so frequently.

Switching to Extended-Release Formulations

If your fatigue seems tied to GI side effects, dose timing, or the peaks and troughs of the standard immediate-release tablet, switching to an extended-release (XR) formulation can help. Research comparing the two has found that extended-release metformin delivers similar blood-sugar control with fewer gastrointestinal side effects and what patients describe as a greater sense of well-being.¹⁵ A systematic review and meta-analysis also looked at the comparison between formulations and found evidence supporting the switch from twice-daily immediate-release to once-daily extended-release as a viable strategy for improving tolerability.¹⁶

The extended-release version releases the drug more slowly, which smooths out the GI irritation that comes from a large dose hitting the stomach lining all at once. It also simplifies dosing to once daily, which some people find easier to maintain. If you’re currently on the immediate-release form and struggling with side effects that may be contributing to your fatigue, asking your prescriber about a switch is a reasonable first step.

A Practical Approach to Sorting Out Your Fatigue

Because metformin-related fatigue has several possible causes, the best strategy is to work through them methodically rather than guessing. Here’s a reasonable order of operations:

Check B12 levels: A simple blood test can reveal whether B12 deficiency is part of the picture. If levels are low, supplementation (oral or injected) usually brings energy back within weeks to months. Adding calcium with your metformin dose can help prevent the problem from recurring.
Review kidney function: Your prescriber should already be monitoring this, but confirm that your most recent blood work includes a kidney panel. Declining kidney function changes how metformin accumulates and affects you.
Track blood sugar patterns: If your fatigue has an episodic or post-meal pattern, consider checking blood sugar during symptomatic periods. Even occasional hypoglycemic dips can produce chronic tiredness.
Assess GI symptoms honestly: Persistent diarrhea that you’ve been tolerating as a minor inconvenience may be causing fluid and electrolyte losses that contribute to fatigue. A basic metabolic panel can check for this.
Consider the formulation: If you’re on immediate-release metformin and experiencing GI issues, a switch to extended-release may reduce side effects and the fatigue that comes with them.
Screen thyroid function: Especially if you have other symptoms like weight changes, cold intolerance, or dry skin, make sure your thyroid isn’t independently contributing to your tiredness.

It’s also worth acknowledging that diabetes itself causes fatigue through blood sugar swings, insulin resistance, and the cardiovascular burden of the disease. Metformin may be one piece of a larger puzzle, and sometimes the fatigue is from the condition rather than the treatment. Separating the two can be tricky, which is why the systematic approach above is more productive than simply blaming the pill.

When to Talk to Your Doctor About Stopping or Changing Metformin

Most of the time, the causes of metformin-related fatigue are manageable without discontinuing the drug. B12 supplementation, a formulation switch, or electrolyte correction can resolve the issue while preserving the metabolic benefits of the medication. But there are situations where stopping or switching makes sense. If you’ve addressed B12, electrolytes, thyroid, and blood sugar and you’re still exhausted, a trial off metformin (with your doctor’s guidance and an alternative blood-sugar plan in place) can clarify whether the drug is the culprit. The case report mentioned earlier, where a patient’s severe daytime fatigue resolved completely within two weeks of stopping metformin, illustrates how clear the answer can sometimes be.⁹

Similarly, if you develop signs that suggest lactic acidosis, particularly sudden worsening of fatigue with nausea, rapid breathing, or muscle pain, metformin should be stopped immediately and you should seek emergency care. And if your kidney function has declined significantly since you started the drug, your prescriber may reduce the dose or switch you to another medication class entirely, since the kidneys are the main route for clearing metformin from your system.¹³ The goal is always a treatment plan where the benefits clearly outweigh the burden, and sometimes that means adjusting rather than enduring.

1
PubMed Central. Metformin-Induced Mitochondrial Complex I Inhibition: Facts, Uncertainties, and Consequences
2
eLife. Metformin inhibits mitochondrial complex I of cancer cells to reduce tumorigenesis
3
Biochemical Journal. Effects of metformin and other biguanides on oxidative phosphorylation in mitochondria
4
PubMed. Effect of metformin on exercise capacity in metabolic syndrome
5
PubMed Central. Association between metformin and vitamin B12 deficiency in patients with type 2 diabetes
6
PubMed. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin
7
PubMed. Effect of calcium supplementation on reversing metformin-based inhibition of vitamin B12 bioavailability in healthy adults
8
PubMed Central. Evaluating Physician Knowledge, Attitudes, and Practices in Screening and Supplementation for Vitamin B12 Deficiency in Type 2 Diabetes Patients Treated with Metformin
9
PubMed Central. Symptomatic Hypoglycemia During Treatment with a Therapeutic Dose of Metformin
10
PubMed Central. An old friend turned foe: Metformin-induced diarrhea with resultant symptomatic hypokalemia, hypomagnesemia, hypocalcemia, and hypophosphatemia
11
PubMed Central. A case report of metformin-associated lactic acidosis
12
PubMed Central. Metformin toxicity in the intensive care unit: A case series and review of the literature
13
PubMed Central. Effect of metformin on kidney function in patients with type 2 diabetes mellitus and moderate chronic kidney disease
14
PubMed Central. Effect of metformin on thyroid stimulating hormone and thyroid volume in patients with prediabetes
15
PubMed Central. Assessment of efficacy and tolerability of once-daily extended release metformin in patients with type 2 diabetes mellitus
16
PubMed Central. Efficacy and Side Effect Profile of Different Formulations of Metformin: A Systematic Review and Meta-Analysis