Metformin, Vitamin B12 and Fatigue: Could Low B12 Be Part of the Problem?
You take metformin to help manage your Type 2 diabetes.
But lately, you're exhausted.
Maybe you're waking up tired. Your energy disappears halfway through the day. Perhaps you've even noticed tingling or numbness in your hands or feet.
So naturally, you start wondering:
“Is metformin making me tired?”
Maybe—but there is an important distinction.
Metformin itself shouldn't automatically be blamed for every case of fatigue.
However, long-term metformin use is associated with lower vitamin B12 levels, and vitamin B12 deficiency can cause fatigue, weakness, anemia, and neurological symptoms. Current American Diabetes Association guidance recommends paying attention to B12 status in people taking metformin chronically, particularly with longer treatment duration, higher doses, or symptoms that could suggest deficiency.
The useful question is not:
“Should I stop metformin?”
It's:
“Could low vitamin B12 be one possible reason I'm tired—and is it worth getting checked?”
The Short Answer
Yes, it could be.
Vitamin B12 deficiency can cause:
Fatigue
Weakness
Anemia
Pale skin
Heart palpitations
A sore or inflamed tongue
Numbness or tingling in the hands and feet
Other neurological changes
Importantly, neurological symptoms from B12 deficiency can sometimes occur even without anemia.
Metformin can reduce vitamin B12 levels, particularly with longer treatment duration and higher doses. The UK Medicines and Healthcare products Regulatory Agency now considers reduced B12 a common adverse effect of metformin treatment and advises testing when deficiency is suspected.
But fatigue is extremely nonspecific.
You cannot determine your B12 level from tiredness alone.
The answer comes from looking at your risk factors, symptoms, medication history, and—when appropriate—blood testing.
Why Does Vitamin B12 Matter?
Vitamin B12 helps support healthy blood cells and normal nerve function.
When the body becomes deficient, it may not produce healthy blood cells normally, and the nervous system can also be affected. That's why B12 deficiency may show up as both fatigue and symptoms such as numbness or tingling.
Here's a useful way to think about it.
Imagine your body as a delivery system.
Your blood carries what tissues need, while your nerves act like communication wires.
Vitamin B12 helps support both systems.
If B12 becomes genuinely deficient, you may start noticing problems in more than one place:
Energy → Blood → Nerves
That's why fatigue isn't the only clue worth noticing.
How Can Metformin Affect Vitamin B12?
Metformin has been associated with reduced absorption of vitamin B12 and lower blood B12 concentrations. The exact mechanism appears to involve several factors rather than one simple pathway.
And the effect usually isn't something that happens overnight.
The body stores substantial amounts of vitamin B12, which is one reason deficiency symptoms may take years to become noticeable.
This helps explain why someone may say:
“I've taken metformin for years without a problem. Why would B12 suddenly matter now?”
Because this is generally a long-term issue, not necessarily an immediate medication reaction.
Who Is More Likely to Develop Low B12 While Taking Metformin?
Risk isn't the same for everyone.
The ADA's 2026 Standards of Care says periodic B12 assessment should be considered in people taking metformin chronically, especially with:
Higher metformin doses, such as 1,500 mg/day or more
Longer treatment duration, particularly around 4–5 years or longer
Other risk factors for B12 deficiency
Anemia
Peripheral neuropathy
Chronic kidney disease
Symptoms that raise suspicion for deficiency
The ADA's guidance for older adults is even more specific: for people taking metformin long term—more than four years—annual B12 monitoring is recommended in that population.
That does not mean every person taking metformin needs to panic about B12.
It means this is a known issue worth remembering.
The Symptom Overlap Problem
This may be the most important part of this article.
Imagine someone with Type 2 diabetes who develops:
Fatigue + tingling in the feet.
The obvious conclusion might be:
“My diabetes is getting worse.”
But B12 deficiency can also produce fatigue and neurological symptoms such as numbness and tingling.
That does not mean the symptoms are caused by B12.
It means there may be more than one possible explanation.
This is exactly why guessing can get people into trouble.
Instead of assuming:
“It's diabetic neuropathy.”
or:
“It's definitely metformin.”
the safer question is:
“Should vitamin B12 be one of the things we check?”
That's useful information to bring to your healthcare professional.
The Health & Fitt 5-Part B12 Clue Check
This is an educational framework—not a diagnostic test.
Remember:
Time → Dose → Symptoms → Other Risks → Test
1. Time — How Long Have You Been Taking Metformin?
A few weeks and several years are very different situations.
Longer metformin use is associated with greater risk of low B12.
You don't need to remember the exact start date.
Even knowing:
“I've been taking it for about six years.”
is useful information.
2. Dose — What Dose Are You Taking?
Higher doses appear to increase the risk.
ADA specifically highlights doses around 1,500 mg per day or higher as one reason clinicians may pay closer attention to B12 status.
Do not lower your dose because of this article.
Just know your dose when discussing the issue with your healthcare professional.
3. Symptoms — Is Fatigue the Only Clue?
Fatigue by itself is weak evidence.
Fatigue plus other possible B12 symptoms gives the question more context.
Look for things such as:
New or worsening tiredness
Weakness
Pale skin
Sore or inflamed tongue
Numbness
Tingling
Balance or neurological changes
These symptoms can have many causes, so they still do not prove B12 deficiency.
4. Other Risks — Could Something Else Be Lowering B12?
Metformin isn't the only reason people become B12 deficient.
Other risk factors include difficulty absorbing B12, pernicious anemia, gastrointestinal surgery or disease, older age, vegetarian or vegan eating patterns, and prolonged use of some acid-suppressing medications.
Sometimes several risk factors stack together.
5. Test — Don't Diagnose B12 Deficiency From Symptoms
A blood test is usually the starting point for evaluating B12 status.
Serum or plasma B12 is commonly measured. If the result is borderline, clinicians may sometimes use a test called methylmalonic acid, or MMA, to help clarify whether deficiency is present.
The point isn't to memorize laboratory ranges.
The point is simple:
Symptoms raise the question. Testing helps answer it.
Should Everyone Taking Metformin Take a B12 Supplement?
No.
This is where health information can easily turn into supplement marketing.
Vitamin B12 is often sold as an “energy vitamin.”
But NIH says B12 supplementation does not appear to improve performance or endurance in people who already have sufficient B12 status.
So the Health & Fitt position should be:
Correct a deficiency—not chase an energy boost.
If you're deficient, treatment matters.
If your B12 level is adequate, blindly taking more B12 should not be expected to magically fix fatigue.
That's an important distinction.
What If My B12 Is Low?
Treatment depends on the cause and severity of the deficiency.
Vitamin B12 deficiency can be treated with oral B12 or injections depending on the individual's situation, including how well they absorb the vitamin and the severity of deficiency.
Your healthcare professional may also need to determine why the level is low.
Metformin may be part of the explanation.
But there could be another cause.
And this is crucial:
Do Not Stop Metformin Because Your B12 Is Low
Regulatory guidance specifically advises that B12 deficiency can be corrected while metformin is continued when the medication remains appropriate and tolerated. Patients are advised not to stop metformin without first discussing it with their doctor.
That's very different from what some internet headlines imply.
The answer usually isn't:
“Metformin stole my B12. Stop the drug.”
The better approach is:
Identify the deficiency → treat it appropriately → continue managing diabetes with your healthcare team.
What If I'm Taking Metformin and I'm Tired—but My B12 Is Normal?
Then don't force B12 to be the answer.
Remember the fatigue cluster we've been building.
Fatigue with Type 2 diabetes may also involve:
High blood sugar
Low blood sugar
Poor sleep
Sleep apnea
Dehydration
Other medications
Anemia unrelated to B12
Thyroid problems
Other health conditions
Your broader guide covers the bigger picture:
Why Am I Always Tired With Type 2 Diabetes?
And if fatigue seems to happen specifically alongside higher glucose readings:
Does High Blood Sugar Make You Tired?
The goal of this article isn't to turn B12 into the explanation for everything.
It's to make sure a real, testable possibility doesn't get overlooked.
A Simple Conversation to Have With Your Doctor
You don't need to walk into the office diagnosing yourself.
Try something like:
“I've been taking metformin for several years and I've been unusually tired lately. Should we check my vitamin B12 level?”
If you've also developed tingling, numbness, anemia, or other symptoms, mention those too.
That conversation is specific.
It's useful.
And it gives your healthcare professional something concrete to evaluate.
Not sure when fatigue deserves medical attention? Read our guide to "when you should talk to a doctor about diabetes fatigue".
Frequently Asked Questions
Can metformin cause vitamin B12 deficiency?
Long-term metformin treatment is associated with reduced B12 levels and an increased risk of deficiency. Risk is greater with higher doses, longer treatment duration, and other B12 risk factors.
Can low vitamin B12 make you tired?
Yes. Fatigue and weakness are recognized symptoms of B12 deficiency. Deficiency can also cause anemia and neurological symptoms.
How long do you have to take metformin before B12 becomes a concern?
There isn't one exact cutoff for everyone. Risk increases with duration. ADA specifically highlights longer-term use around 4–5 years or more, particularly when combined with higher doses or other risk factors.
Can B12 deficiency cause tingling in the hands and feet?
Yes. Vitamin B12 deficiency can cause neurological changes including numbness and tingling. These symptoms can sometimes appear even without anemia.
Should I stop metformin if my B12 is low?
Not on your own. Guidance says B12 deficiency can generally be corrected while continuing metformin when the medication remains appropriate. Discuss any medication changes with your healthcare professional.
Will taking B12 give me more energy?
If B12 deficiency is contributing to fatigue, correcting the deficiency may address that problem. But NIH notes that extra B12 does not appear to improve performance in people whose B12 status is already adequate.
The One Thing to Remember
If you take metformin and you're tired, don't automatically conclude:
“Metformin is making me tired.”
Instead remember:
Time → Dose → Symptoms → Other Risks → Test
Metformin can lower vitamin B12 over time.
Low B12 can cause fatigue.
But fatigue does not prove B12 deficiency.
Check the possibility instead of guessing.
