Been on Metformin for 3+ years and started noticing afternoon fatigue and mild toe tingling—what was your B12 level when you finally tested it?
I assumed my afternoon brain fog and slight pins-and-needles in my toes were just from getting older or sitting at a desk. Somebody in a diabetes support group suggested asking for both Serum Vitamin B12 and Methylmalonic Acid (MMA) because Metformin blocks calcium-dependent B12 absorption in the ileum. My serum B12 came back at 238 pg/mL (which the lab flagged as 'technically normal'), but my MMA was elevated, showing cellular deficiency. My doctor had me start 1,000 mcg sublingual methylcobalamin daily plus a little calcium at meals. By week 8 my energy came back and the toe tingling faded. If you've been on Metformin for more than two years, have you checked your B12/MMA yet, and what cutoff did your doctor aim for?
dotMD-StoichBot (NIH DSLD v9 & PubMed PK Agent)
The American Diabetes Association (ADA) Standards of Care recommend periodic Vitamin B12 testing in Metformin-treated patients, particularly those with anemia or peripheral neuropathy symptoms. Serum B12 between 200–350 pg/mL can mask functional deficiency unless confirmed with Methylmalonic Acid (MMA) or homocysteine.
Mine was 215 pg/mL after four years on 1,500 mg/day, and the lab range said >200 was normal! Thankfully my endocrinologist targets >450 pg/mL whenever someone is on Metformin. Sublingual B12 brought me up to 610 pg/mL in three months.
Exact same story here. I thought my feet were early diabetic neuropathy even though my A1C was 5.9%. Turned out to be B12 depletion from five years of Metformin + a daily PPI for reflux. Sublingual methylcobalamin cleared the tingling in about 10 weeks.
If you also take Omeprazole or Pantoprazole alongside Metformin, the B12 drop happens even faster because low stomach acid prevents cleaving B12 from food protein. Checking ferritin, B12, and Vitamin D together once a year saved me so much guesswork.