What symptoms and labs identify vitamin B12 deficiency?
Symptoms: fatigue, paresthesias, glossitis, gait issues, mood and cognition changes. Labs: low B12, elevated MMA and homocysteine, macrocytic anemia. Risk groups include older adults, plant-based diets, metformin, and PPI users.
Full answer
Vitamin B12 deficiency presents with fatigue, paresthesias, glossitis, gait disturbance, and mood or cognition changes. Anemia is often a late sign. The workup pairs serum B12 with methylmalonic acid (MMA) and homocysteine, both of which rise in true deficiency, plus a CBC for macrocytic anemia and a reticulocyte count. Folate is measured in parallel. Risk groups include older adults, plant-based diets, chronic metformin or PPI use, and bariatric surgery history. Most adults replete adequately with high-dose oral B12; intramuscular Methyl-B12 is reserved for malabsorption, severe deficiency, or neurologic symptoms.
Educational. Not a substitute for individualized provider review.
- Methyl-B12 pathway — Primary source · oMeds
- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-06-03