Metformin is one of the most common drugs used to treat type 2 diabetes. It’s cheap, effective, and has been used for decades. But for people who take it for years - especially over five or more - there’s a hidden risk that many doctors still don’t talk about: vitamin B12 deficiency. This isn’t a rare side effect. It’s common. And if left unchecked, it can cause serious, sometimes permanent damage to your nerves, brain, and blood.

How Metformin Steals Your B12

You don’t get vitamin B12 from your body. You get it from food - meat, eggs, dairy, fish. But your body can’t absorb it without help. A complex process happens in your small intestine, where a protein called intrinsic factor binds to B12, and calcium helps it stick to receptors in the ileum. That’s where metformin interferes.

Studies show metformin blocks calcium’s role in this process. Without enough calcium at the right spot, B12 just slides right through your gut and out of your body. This isn’t a minor issue. Research from the Diabetes Prevention Program found that for every year you take metformin, your risk of B12 deficiency goes up by 13%. At 10 years or more, nearly half of users have low levels.

It gets worse if you’re on other medications. Proton-pump inhibitors (PPIs) like omeprazole, used for heartburn, cut stomach acid. That acid is needed to free B12 from food. So if you’re taking both metformin and a PPI, your body has a double hit. One study found that 40% of people with type 2 diabetes on both drugs had B12 levels below normal.

Why This Is Silent and Dangerous

Your liver stores about 2,500 micrograms of B12. You only need 2.4 micrograms a day. So it takes years - sometimes a decade - for your stores to run low. That’s why most people don’t feel anything at first. No dizziness. No fatigue. Just quiet, slow damage.

By the time symptoms show up, it’s often too late. Nerve damage from B12 deficiency doesn’t always reverse. You might start with tingling in your hands or feet - but your doctor might blame it on diabetic neuropathy. You feel tired? That’s just diabetes, right? A sore tongue? Maybe you’re stressed. These are the same symptoms of type 2 diabetes. So the real cause gets missed.

One patient, who posted on the NHS forum after eight years on metformin, had severe nerve pain and couldn’t walk without help. Her B12 level was 128 pmol/L - far below the normal range of 221+. After six months of B12 shots, her symptoms improved dramatically. She wasn’t alone. Reddit threads are full of people who spent years with numbness, memory fog, or balance problems - only to find out their B12 was dangerously low.

Who’s at Highest Risk?

Not everyone on metformin gets deficient. But some groups are far more vulnerable:

• People on high doses - over 2,000 mg per day
• Those on metformin for more than four years
• Vegans and vegetarians - they get no B12 from food, so their reserves run out faster
• People on PPIs or H2 blockers - these drugs block stomach acid needed to release B12
• Older adults - natural B12 absorption drops with age
• People with digestive disorders - Crohn’s, celiac, or past gastric surgery

One study found that vegetarians on metformin had a 78% higher chance of B12 deficiency than non-vegetarians. That’s not a coincidence. It’s a perfect storm.

What Symptoms Should You Watch For?

These aren’t just random complaints. They’re red flags:

• Constant fatigue - even after good sleep
• Muscle weakness - especially in legs or arms
• Tingling, burning, or numbness in hands or feet
• Sore, red tongue or mouth ulcers
• Difficulty balancing or walking straight
• Blurred or double vision
• Mood changes - depression, irritability, brain fog
• Pale or yellowish skin

Here’s the kicker: 38% of people in one clinical study had neurological symptoms before any signs of anemia. That means you could be damaging your nerves long before a blood test shows low B12. If you’ve been on metformin for five years and have any of these, get tested - don’t wait.

Testing and Diagnosis: What Your Doctor Should Do

Just checking serum B12 isn’t enough. Your body can fake normal levels even when your tissues are starving. That’s why experts now recommend checking two things:

• Serum B12 - the standard blood test
• Methylmalonic acid (MMA) - a marker that rises when B12 is low in tissues

If your B12 is borderline (between 200 and 300 pg/mL), MMA tells you if you’re really deficient. Homocysteine is another option - it also rises with B12 shortage. But MMA is more specific.

The European Association for the Study of Diabetes says: test B12 at baseline and every two to three years for everyone on metformin. The American Diabetes Association says “consider” testing - but that’s vague. If you’ve been on it for five years, don’t wait for your doctor to bring it up. Ask for it.

What to Do If You’re Deficient

It’s treatable. But speed matters. The longer you wait, the higher the chance of permanent nerve damage.

For mild deficiency: High-dose oral B12 - 1,000 to 2,000 mcg daily - works well. Studies show 89% of people recover within three months.

For moderate to severe: Injections are faster. A common protocol is 1,000 mcg intramuscular shots once a week for four weeks, then monthly. Many patients report feeling better within weeks.

Don’t stop metformin. You still need it to control your blood sugar. But you can fix the B12 issue alongside it.

A Surprising Prevention Trick

Here’s something most people don’t know: taking calcium might help prevent B12 loss.

A 2021 randomized trial gave metformin users 1,200 mg of calcium carbonate daily. After two years, those taking calcium had a 47% lower chance of becoming deficient. Why? Calcium helps the B12-intrinsic factor complex bind to receptors - the exact step metformin blocks. So if you take metformin long-term, adding a calcium supplement might be one of the smartest moves you make.

Look for calcium citrate or carbonate - 1,200 mg per day is the dose studied. Talk to your doctor before starting, especially if you have kidney issues.

The Bigger Picture

Over 150 million people worldwide take metformin. Studies suggest 10-30% of long-term users are deficient. That’s millions of people at risk for nerve damage, cognitive decline, and anemia - all preventable with simple blood tests and cheap supplements.

The UK Medicines and Healthcare products Regulatory Agency now lists B12 deficiency as a common side effect. The FDA updated metformin’s label in 2022. The European Medicines Agency did the same in 2021. This isn’t theory anymore. It’s policy.

And the cost of ignoring it? The UK’s NHS spends £47 million a year treating complications from undiagnosed B12 deficiency in metformin users. In the U.S., modeling shows that testing every patient on long-term metformin saves $142-$187 per person by avoiding expensive nerve treatments and hospital visits.

Emerging research even points to genetics. Some people have a variant in the CUBN gene - the one that codes for the B12 receptor. Those people are far more likely to lose B12 on metformin. Future tests might screen for this before prescribing.

Your Action Plan

If you’ve been on metformin for four or more years, here’s what to do:

1. Ask your doctor for a serum B12 test. If it’s below 300 pg/mL, request a methylmalonic acid (MMA) test.
2. If you’re vegetarian or vegan, ask for testing even if you’ve been on it for less than four years.
3. If you take PPIs (like omeprazole), get tested now - don’t wait.
4. Consider taking 1,200 mg of calcium daily - it’s cheap, safe, and backed by science.
5. If your B12 is low, start treatment immediately - oral supplements or injections, as advised.
6. Repeat testing every two to three years, even if you feel fine.

This isn’t about fear. It’s about awareness. Metformin saves lives. But like all medicines, it has trade-offs. Knowing about this risk doesn’t mean you should stop taking it. It means you should take charge - ask the right questions, get tested, and protect your long-term health.