Should B12 be checked when I am tired?
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Short answer
Vitamin B12 is worth checking when fatigue comes with numbness, tingling, balance problems or memory change, and in anyone taking metformin or long term acid suppressing drugs. Most laboratories call a serum B12 below 200 to 250 pg/mL subnormal. Between 150 and 399 pg/mL is a grey zone where methylmalonic acid should be measured, since it is the more sensitive marker.
Get urgent care if: you develop any of these neurological changes, which can indicate advancing B12 deficiency.
- Rapidly progressive numbness or weakness in the legs
- Difficulty walking or frequent falls
- New confusion or marked memory change
- Loss of bladder or bowel control
Vitamin B12 deficiency is worth finding because the neurological damage it causes can become permanent. It is also a test that is easy to misread, because the serum B12 level is an imperfect marker and a result in the low-normal range does not settle the question. The NIH Office of Dietary Supplements sets out the approach: measure serum B12 first, then use methylmalonic acid when the result falls in the uncertain middle.
What the numbers mean
The NIH fact sheet notes that most laboratories define subnormal serum or plasma values as those lower than 200 or 250 pg/mL. It also states that methylmalonic acid levels "are the most sensitive markers of vitamin B12 status, and an MMA level greater than 0.271 micromol/L suggests vitamin B12 deficiency". The practical rule it gives is this: if a serum B12 sits between 150 and 399 pg/mL, check methylmalonic acid to help confirm the diagnosis. A homocysteine above 15 micromol/L also suggests deficiency, but it is less specific because folate status and kidney function both affect it.
Who is actually at risk
The NIH fact sheet states that metformin "might reduce the absorption of vitamin B12 and significantly reduce serum vitamin B12 concentrations", and that acid suppressing drugs used for reflux and ulcers can interfere with B12 absorption from food. It also identifies older adults with atrophic gastritis, a condition affecting 8 to 9 percent of adults aged 65 and over, people following vegan or vegetarian diets, and anyone who has had gastric or bariatric surgery. That covers a large share of the patients I see in a metabolic practice.
Symptoms that should prompt testing
Fatigue alone is a weak reason to test B12, because it is the least specific symptom. The findings that should trigger testing are numbness or pins and needles in the feet or hands, unsteadiness in the dark, a sore smooth tongue, memory or concentration change, and unexplained macrocytosis on a blood count. Neurological features can appear before anaemia does, so a normal haemoglobin does not rule deficiency out.
| Serum B12 | Interpretation | Next step |
|---|---|---|
| Below 150 pg/mL | Deficiency likely | Treat and find the cause |
| 150 to 399 pg/mL | Grey zone | Measure methylmalonic acid |
| MMA above 0.271 micromol/L | Deficiency suggested | Treat; consider kidney function and age |
| Above 400 pg/mL without symptoms | Deficiency unlikely | Look elsewhere for the cause |
If you take metformin, ask for a B12 level rather than waiting for symptoms. If you have numbness or balance change with a low-normal B12, ask for methylmalonic acid before the question is dropped. Do not start high dose B12 before the blood is drawn, since supplementation makes the result uninterpretable for months.
The clinical detail
Methylmalonic acid rises in renal impairment and with age, so an isolated mild elevation in an older patient with reduced eGFR is interpreted cautiously alongside homocysteine. Folate supplementation can correct the megaloblastic anaemia of B12 deficiency while the neurological damage continues, which is why B12 status is established before folate replacement in a macrocytic anaemia.
Cause matters for the route of treatment. Pernicious anaemia, confirmed with intrinsic factor antibodies, and post-surgical malabsorption generally require parenteral or high dose oral therapy indefinitely. Dietary deficiency in a vegan diet responds to oral supplementation. Serum B12 becomes uninterpretable for months after supplementation begins, so testing before treatment is preferable where symptoms allow.
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Sources
- Vitamin B12: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements
- Vitamin B12 deficiency anemia. MedlinePlus Medical Encyclopedia, National Library of Medicine
- Anemia. MedlinePlus, National Library of Medicine
- Diagnosis and Classification of Diabetes: Standards of Care in Diabetes. American Diabetes Association
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