Does Taking Metformin Really Cause B12 Deficiency?

Long-term metformin use causes vitamin B12 deficiency and can lead to nerve damage.

Multiple studies — including a 2019 Diabetes Care meta-analysis of 29 RCTs — confirm that metformin reduces B12 absorption in 10–30% of long-term users by interfering with calcium-dependent ileal membrane B12-intrinsic factor complex uptake. The reduction is dose-dependent and more pronounced at doses above 1,500 mg/day.

This claim is well-supported. If you take metformin, ask your doctor about annual B12 monitoring — especially if you notice numbness, fatigue, or memory issues. Supplementation with crystalline B12 (not just dietary B12) is often recommended. The good news: this side effect is easy to detect and reverse with supplementation.

• [1] diabetesjournals.org
• [2] pubmed.ncbi.nlm.nih.gov