Metformin Long-Term Use: Benefits, Risks, and What You Need to Know

When you take metformin, a first-line oral medication for type 2 diabetes that lowers blood sugar by improving insulin sensitivity and reducing liver glucose production. Also known as Glucophage, it’s one of the most prescribed drugs in the world because it’s cheap, effective, and generally safe—even for long-term use. But taking it for years isn’t risk-free. Many people don’t realize that after 5+ years, metformin can lower vitamin B12 levels by up to 30%, which may lead to nerve damage, fatigue, or even anemia if left unchecked. The FDA doesn’t require routine B12 testing for metformin users, but doctors who’ve seen the data now recommend checking levels every 2–3 years, especially if you’re over 60 or have digestive issues.

Metformin extended-release, a slower-acting form of metformin designed to release the drug gradually through the digestive tract. Also known as metformin XR, it’s easier on the stomach and reduces diarrhea and cramping by up to 50% compared to the immediate-release version. That’s why most new prescriptions now start with XR—it’s not just about comfort, it’s about sticking with the treatment. Long-term users who switch to XR often report better adherence, fewer dropped doses, and less nausea. But even with XR, your body still absorbs less B12 over time, and your gut microbiome changes. Studies show metformin alters gut bacteria in ways that may help control blood sugar, but could also increase gas and bloating. These aren’t side effects you can just ignore—they’re signals your body is adapting.

People with PCOS, a hormonal disorder affecting up to 1 in 10 women of reproductive age, often marked by insulin resistance, irregular periods, and elevated testosterone. Also known as polycystic ovary syndrome, it’s one of the most common off-label uses for metformin use it for years to restart ovulation and lower male hormones. Unlike in diabetes, where the goal is blood sugar control, in PCOS, metformin helps reset the body’s metabolic response. Many women see their cycles return, acne improve, and even lose weight—without needing fertility drugs. But again, long-term use means monitoring. Low B12 can mimic PCOS symptoms like brain fog and fatigue, so treating one condition might mask the other.

There’s no magic number for how long you should take metformin. Some people take it for decades. Others stop after their blood sugar normalizes. The key isn’t the duration—it’s the awareness. If you’ve been on metformin for more than two years, ask your doctor for a B12 test. If you’re still having stomach issues, ask about switching to XR. If you’re using it for PCOS, track your cycles and hormone levels. This isn’t a drug you take and forget. It’s a tool that works best when you stay informed.

Below, you’ll find real stories and science-backed insights from people who’ve lived with metformin long-term—what worked, what didn’t, and what they wish they’d known earlier.