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

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. Yet, despite its widespread use, confusion around metformin is everywhere. You hear stories about people quitting because of stomach issues. You read warnings about vitamin B12 dropping. You wonder if it’s safe to take for decades. Let’s cut through the noise. Here’s what actually happens when you take metformin long-term - and what’s just a myth.

Myth: Metformin Always Gives You Bad Stomach Problems

It’s true - many people get nausea, diarrhea, or bloating when they start metformin. In fact, about 28% of people in early clinical trials reported these symptoms. But here’s what no one tells you: most of those side effects fade.

The Diabetes Prevention Program followed over 3,000 people for 15 years. At first, those on metformin had twice as many GI issues as those on placebo. But by year five, the difference vanished. By year ten, the rates were nearly identical. People didn’t magically become immune - they adapted. Their bodies adjusted. Their gut learned to handle it.

The real issue? Starting too fast. Taking 1,000 mg on day one is asking for trouble. The standard protocol - start with 500 mg once a day with dinner, then increase by 500 mg every week - cuts discontinuation rates in half. One 2023 study showed dropping from 15% to just 5% when people took it slow.

And if you’re still struggling after a few months? Switch to extended-release (XR). XR metformin releases the drug slowly, so your gut isn’t hit with a full dose all at once. One chart review found that when patients switched from regular metformin to XR, diarrhea dropped from 18% to 8%. Abdominal pain fell by more than half. People who thought they couldn’t tolerate metformin often just needed the right version.

Myth: You Can’t Take Metformin for More Than a Few Years

There’s a quiet fear that if you take metformin too long, something bad will happen. That it wears out your liver. That it damages your kidneys. That it’s a temporary fix.

The truth? Metformin has been used for over 60 years. The longest-running study - the Diabetes Prevention Program Outcomes Study - tracked people for 15 years. No new safety red flags popped up. No spike in cancer. No hidden organ damage. No mysterious decline in health.

The only real risk? Lactic acidosis. But this isn’t something that happens to healthy people. It’s extremely rare - 3 to 10 cases per 100,000 patient-years. And it almost always occurs in people with severe kidney disease (eGFR under 30), liver failure, or during serious illness like sepsis or heart attack. If your kidneys are working normally, you’re not at risk.

Even the slight drop in hemoglobin and hematocrit seen in the first year? It stabilizes. Your body adjusts. No further decline happens. Metformin doesn’t wear out your system. It works with it.

And here’s the kicker: metformin helps you keep weight off. In the same 15-year study, people on metformin lost 2-3% of their body weight and kept it off. Compare that to insulin or sulfonylureas - they make you gain weight. Metformin doesn’t just control blood sugar. It helps you stay healthier overall.

Myth: Metformin Doesn’t Affect Vitamin B12

This is one of the most overlooked facts. If you’ve been on metformin for more than four years, your vitamin B12 levels are likely lower than they should be.

A 2020 review of 14 studies found that long-term metformin users had, on average, a 19% drop in B12 levels compared to non-users. That’s not a small thing. Low B12 can cause fatigue, brain fog, tingling in your hands and feet, and even anemia. It can mimic nerve damage from diabetes - which means doctors might misdiagnose it.

The American Diabetes Association now recommends checking B12 levels every 2 to 3 years if you’re on metformin long-term. If you have numbness, weakness, or unexplained fatigue, get tested. It’s simple. A blood test. If your levels are low, you can take an oral supplement or a B12 injection. It’s cheap, safe, and fixes the problem.

Don’t panic. This isn’t a reason to stop metformin. It’s a reason to be smart. Most people never know their B12 is low because no one checks. But if you’re on metformin for five years or more, it’s part of your care. Just like checking your kidney function or your A1C.

Myth: Newer Diabetes Drugs Are Better Than Metformin

GLP-1 agonists like Ozempic and Mounjaro get all the attention. They cause big weight loss. They protect the heart. They sound amazing. So why isn’t everyone switching?

Because metformin still wins on three things: cost, safety, and long-term data.

Metformin costs $4 to $10 a month. A month of Ozempic? Over $900 without insurance. Even with discounts, it’s still 50 to 100 times more expensive. And while GLP-1s are great for weight and heart health, we don’t have 15-year safety data on them yet. We do on metformin.

The American Diabetes Association, the European Association for the Study of Diabetes, and the American College of Physicians all still call metformin the first-line treatment. Why? Because no other drug has matched its combination of effectiveness, safety, and affordability over decades.

Newer drugs aren’t replacements. They’re options - especially for people who need more weight loss or have heart disease. But for most people starting type 2 diabetes, metformin is still the best first step.

What Works in Real Life

People on forums like Reddit and Diabetes Daily share what actually helps:

• Start low: 500 mg once a day with dinner.
• Go slow: Increase by 500 mg every week.
• Switch to XR: Once-daily, bedtime dose cuts side effects by 30%.
• Take with food: Always. Never on an empty stomach.
• Check B12: Every 2-3 years after year four.
• Don’t quit too soon: Most GI issues fade by month three.

One Reddit user, Type2Warrior87, said: "Started on 500mg IR, had terrible diarrhea for two weeks. Switched to 500mg XR once daily with dinner - zero issues after six months." That’s the story for most people who stick with it.

Another user, MJS99, had a different experience: "XR worked for three months, then nausea came back. I had to stop." That’s the minority. But it’s real. If you’ve tried everything and still can’t tolerate it, talk to your doctor. There are other options. But don’t assume metformin is the problem - often, it’s just the timing or the form.

Bottom Line

Metformin isn’t perfect. But it’s the most studied, safest, and cheapest diabetes drug we have. The myths? They’re louder than the facts. GI issues? Usually temporary. B12 drop? Easy to fix. Long-term use? Proven safe for decades.

If you’re on metformin and worried, don’t guess. Get your B12 checked. Ask about switching to XR. Talk about your side effects - don’t just quit. Most people who stick with it end up feeling better, not worse.

Metformin doesn’t cure diabetes. But it gives you time. Time to lose weight. Time to change your diet. Time to prevent complications. And for millions of people, that’s worth sticking with.

Still unsure? Talk to your doctor. Bring this information. Ask about your B12. Ask about switching to XR. Ask if your side effects are normal. You’re not alone - and you don’t have to suffer through side effects that can be fixed.