Metformin and B12 Deficiency: What You Need to Know

When you take metformin, a first-line medication for type 2 diabetes that helps lower blood sugar by reducing liver glucose production and improving insulin sensitivity. It's one of the most prescribed drugs in the world—but it doesn’t come without hidden side effects. One of the least talked about is its link to vitamin B12 deficiency, a condition where your body lacks enough B12 to make healthy red blood cells and maintain nerve function. Also known as cobalamin deficiency, this isn’t just about feeling tired—it can cause numbness, memory problems, and even permanent nerve damage if ignored.

Studies show that up to 30% of people on long-term metformin develop low B12 levels. The longer you take it—especially over 4 years—and the higher your dose, the greater the risk. It’s not because you’re not eating enough B12; your body just can’t absorb it properly when metformin is in the picture. It interferes with calcium-dependent B12 uptake in the gut. That means even if you eat salmon, eggs, or dairy daily, your body might still be starving for B12. And here’s the catch: symptoms like fatigue, brain fog, or tingling hands often get blamed on aging, stress, or diabetes itself. By the time doctors test for it, damage may already be done.

That’s why anyone on metformin should get their B12 checked every year—especially if they’re over 60, vegetarian, or have digestive issues like IBS or Crohn’s. It’s simple: a blood test takes minutes, and fixing it with oral supplements or injections can reverse symptoms fast. You don’t need to stop metformin. You just need to stay aware. Below, you’ll find real-world insights from patients and doctors on how this interaction plays out, what alternatives exist, and how to protect your health without giving up the drug that keeps your blood sugar in check.