Metformin Myths and Facts: Tolerability, B12, and Long-Term Use

• by Nikolai Mortenson
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Metformin is the most prescribed diabetes medication in the world. Over 160 million prescriptions are filled for it every year in the U.S. alone. It’s cheap, effective, and has been used for over 60 years. But despite all that, it’s surrounded by myths. People hear it causes lactic acidosis, ruins your stomach forever, or steals your vitamin B12 - and they stop taking it. The truth? Most of these fears are exaggerated, misunderstood, or outright wrong. Let’s clear the air.

Myth: Metformin Is Too Hard on the Stomach - Most People Can’t Tolerate It

It’s true that metformin can upset your stomach. Diarrhea, nausea, bloating - these are common at first. In the Diabetes Prevention Program, nearly 28% of people on metformin had GI issues compared to 16% on placebo. That sounds bad, but here’s what no one tells you: those symptoms fade.

By the end of the first year, most people adapt. In the 15-year follow-up study (DPPOS), GI side effects in the metformin group dropped to match the placebo group. People didn’t just endure it - they got used to it. One Reddit user, Type2Warrior87, said: "Started on 500mg IR, had terrible diarrhea for 2 weeks. Switched to 500mg XR once daily with dinner - zero issues after 6 months."

The key is how you start. Taking a full dose right away? Bad idea. The Diabetes Prevention Program protocol recommends starting at 500mg once a day with your evening meal. Then, increase by 500mg every week. This slow ramp-up cuts discontinuation rates in half - from 15% down to just 4.7%. Most doctors skip this step. Don’t let yours.

And if you’re still struggling? Switch to extended-release (XR). A 2022 study showed that switching from immediate-release to XR cut diarrhea from 18% to 8%. Abdominal pain dropped by more than half. XR lets you take it once a day, usually at bedtime, which reduces stomach irritation. It’s not magic - but it’s science.

Only about 4% of people quit metformin because of side effects. That’s not "most people." That’s a minority. If you’re having trouble, it’s not a reason to stop. It’s a reason to talk to your doctor about timing, dosage, or formulation.

Myth: Metformin Causes Lactic Acidosis - It’s Deadly

Lactic acidosis sounds terrifying. And it is - when it happens. But it almost never happens in healthy people taking metformin. The FDA estimates only 3 to 10 cases per 100,000 patient-years. That’s rarer than being struck by lightning.

Here’s the catch: lactic acidosis doesn’t happen because of metformin. It happens because of other problems - like kidney failure, liver disease, severe infection, or alcohol abuse. Metformin is cleared by the kidneys. If your kidneys aren’t working (eGFR below 30), it builds up. That’s why doctors check your kidney function before prescribing it - and every year after.

The Diabetes Prevention Program tracked over 3,000 people for 15 years. Not one case of lactic acidosis occurred in the metformin group. Zero. In real-world use, the risk is negligible if you’re not in a high-risk group. The fear persists because it’s dramatic. But the data? It’s reassuring.

Myth: Metformin Makes You Gain Weight

Many diabetes drugs make you gain weight. Insulin? Weight gain. Sulfonylureas? Weight gain. Even some newer drugs like GLP-1 agonists are marketed for weight loss - because the alternatives make you heavier.

Metformin does the opposite. In the Diabetes Prevention Program, people on metformin lost 2-3% of their body weight over 10-15 years. That’s not huge, but it’s real. And it’s consistent. While others gained pounds, metformin users held steady or lost a few. That’s why it’s still the first-line choice - it doesn’t add to the problem it’s meant to fix.

It doesn’t suppress appetite like semaglutide. But it reduces liver sugar production and improves how your body uses insulin. That means fewer blood sugar spikes, fewer cravings, and less fat storage. For someone trying to manage type 2 diabetes, that’s a huge advantage.

Fact: Long-Term Metformin Use Can Lower Vitamin B12 Levels

This is the one myth that’s actually true - and it’s often ignored.

Studies show that after 4-12 years of metformin use, vitamin B12 levels drop by about 19% on average. Up to 30% of long-term users develop deficiency. That’s not rare. That’s common.

Why does this matter? Low B12 causes fatigue, nerve damage (neuropathy), memory problems, and anemia. And guess what? Those are also symptoms of diabetes. So if you’re tired, numb in your feet, or foggy-headed, your doctor might blame your diabetes - when it’s actually your metformin.

The American Diabetes Association now recommends checking B12 levels every 2-3 years if you’ve been on metformin for more than 4 years. If you have anemia or nerve symptoms? Check it sooner. It’s a simple blood test. And if your levels are low? A daily B12 supplement - even over-the-counter - can fix it. No need to stop metformin.

Some European guidelines say check every 2 years. The ADA says "periodic." Either way, don’t wait until you’re numb. Get tested. It’s cheap. It’s easy. And it prevents real damage.

Myth: There Are Better Drugs Now - Metformin Is Outdated

Newer drugs like SGLT2 inhibitors and GLP-1 agonists get all the attention. They help with heart health. They cause weight loss. They’re trendy. But that doesn’t make metformin obsolete.

Metformin is still the most prescribed diabetes drug in the world. It’s the first-line choice for the American Diabetes Association, the European Association for the Study of Diabetes, and the American College of Physicians. Why? Because no other drug has matched its combination of safety, effectiveness, cost, and long-term data.

It’s not perfect. But it’s the most studied. The DPP study followed people for 15 years. No other diabetes drug has that kind of track record. SGLT2 inhibitors and GLP-1 agonists have only been around for 10-15 years. We don’t know what happens after 30 years of use. We do know what happens with metformin.

And cost? A 30-day supply of generic metformin costs $4-$10 in the U.S. A month of a GLP-1 drug? $800-$1,000. Insurance doesn’t always cover them. And even if it does, you’re paying more for something that doesn’t necessarily do more.

Metformin isn’t outdated. It’s the foundation. New drugs are often added on top of it - not instead of it.

Myth: If Metformin Doesn’t Work Right Away, It’s Not Working

Metformin doesn’t drop your blood sugar like a rock. It doesn’t give you instant results. It works slowly - over weeks and months. You might not feel different. Your A1c might only drop by 1-2% in the first three months. That’s normal.

It’s not a magic bullet. It’s a tool. It helps your body use insulin better. It reduces liver sugar output. It doesn’t force your pancreas to pump out more insulin. That’s why it doesn’t cause low blood sugar on its own. It’s gentle. But it’s also slow.

Give it time. Stick with the dose. Don’t switch because you didn’t see results in two weeks. That’s not failure. That’s how it works.

What to Do If You’re on Metformin

• Start low. Go slow. 500mg once daily with dinner, then increase weekly.
• If GI issues stick around, ask for the extended-release (XR) version.
• Get your kidney function checked yearly.
• Get your B12 level tested every 2-3 years - or sooner if you have numbness, fatigue, or anemia.
• Don’t stop because of side effects. Adjust, don’t quit.
• Keep taking it even if your blood sugar looks good. It’s not just about glucose - it’s about long-term protection.

Metformin isn’t perfect. But it’s the best we’ve got for most people with type 2 diabetes. The myths are loud. The facts are quiet. Don’t let the noise make you stop taking something that’s saved millions of lives.

What Comes Next?

If you’re on metformin and feeling fine - keep going. If you’re struggling with side effects - talk to your doctor about XR or a slower ramp-up. If you’ve been on it for more than four years - get your B12 checked. If you’re thinking about quitting - ask yourself: are you quitting because of a myth, or a real problem?

Metformin isn’t glamorous. It doesn’t have flashy ads or celebrity endorsements. But it’s been saving lives for decades. And it still is.