My B12 is low — what should I do?
Low B12 is repleted orally for most adults; intramuscular B12 is reserved for malabsorption, severe deficiency, or neurologic symptoms. Methylmalonic acid clarifies borderline values. Folate, MCV, and homocysteine give context.
Full answer
Vitamin B12 deficiency causes macrocytic anemia, fatigue, glossitis, and neurologic symptoms (paresthesias, gait, cognition). Risk groups: older adults, plant-based diets, metformin or PPI use, and bariatric surgery. The workup pairs B12 with folate, MCV, reticulocytes, MMA (clarifies borderline values), and homocysteine. Most adults replete adequately with high-dose oral cyanocobalamin or methylcobalamin. Intramuscular Methyl-B12 is reserved for malabsorption, severe deficiency, or neurologic symptoms and is provider-reviewed. Recheck at 8–12 weeks.
Educational. Not a substitute for individualized provider review.
- Methodology — Primary source · oMeds
- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-06-03