B12 deficiency is a well-established, treatable cause of neuropathy. It is also one of the most commonly mis-cleared.
The problem with serum B12
Serum B12 measures how much is circulating. It does not measure how much is functioning inside cells. The reference range is wide, and the lower portion of it overlaps with people who have demonstrable functional deficiency.
A result of, say, 300 is inside the range. It is also a value at which functional deficiency is entirely possible.
What actually answers the question
Methylmalonic acid. B12 is required to metabolize MMA. When B12 is functionally insufficient, MMA accumulates. Elevated MMA is a more direct indicator of tissue-level status than serum B12.
Homocysteine. Rises in both B12 and folate insufficiency. Interpreted alongside MMA, it helps separate the two.
Who is at higher risk than their labs suggest
Long-term metformin use, which affects B12 absorption. Long-term acid-reducing medication. Prior gastric or intestinal surgery. Celiac or other malabsorption. Vegetarian and vegan diets. Older adults, in whom absorption declines. Nitrous oxide exposure, medical or recreational.
If any of those apply and you have unexplained neuropathy, a normal serum B12 alone is a thin basis for closing the question.
Why timing matters
B12-related nerve injury is more reversible early than late. This is one of the more genuinely time-sensitive items on the neuropathy workup, which is part of why it is worth confirming properly rather than assuming.
Free 15-minute phone consultation. If you have prior B12 results, bring them.
FAQ
What B12 level causes neuropathy?
There is no single cutoff. Functional deficiency can occur at levels inside the standard reference range, which is why MMA and homocysteine are more informative than serum B12 alone.
Can B12 deficiency neuropathy be reversed?
It can improve, particularly when identified early. Long-standing deficiency may leave residual deficits. Duration before correction is the main determinant.
Reviewed by Alvin Philipose, DC, ICCP. This page is for informational purposes only and does not constitute medical advice. Individual results vary and no outcome is guaranteed. If you are experiencing sudden weakness, difficulty breathing or swallowing, new bowel or bladder changes, or a wound or infection on your foot, seek emergency care.