1. Risk Factors for Vitamin B12 Deficiency
Malabsorption
- Atrophic gastritis (including autoimmune gastritis)
- Bariatric surgery — Roux-en-Y highest risk
- Celiac disease
- Crohn disease
- Exocrine pancreatic insufficiency
- Gastrectomy
- Ileal bowel resection
- Tapeworm infestation
Decreased intake of vitamin B12
- Alcohol use disorder
- Allergy to animal products
- Eating disorder
- Food insecurity
- Vegan or vegetarian diet
Medications
- Histamine H₂ blockers
- Metformin
- Nitrous oxide (recreational use)
- Proton pump inhibitors
Testable Pregnancy increases the risk of B12 deficiency, but routine prenatal B12 supplementation is NOT recommended — it adds no benefit.
2. Clinical Features
Cognitive
- Brain fog
- Delirium
- Dementia
- Memory loss
- Mood changes
General
- Fatigue
- Generalized weakness
Hematologic
- Hemolysis
- Macrocytosis
- Megaloblastic anemia
- Pancytopenia
Neurologic
- Ataxia
- Gait disturbance
- Paresthesias
- Peripheral neuropathy
- Proprioception or vibratory-sense loss
- Subacute combined degeneration of the spinal cord
- Vision changes
Psychiatric
- Depression
- Irritability
- Psychosis
Skin
- Glossitis
- Hyperpigmentation
- Jaundice
Testable In severe cases: impaired gait, pancytopenia, cognitive impairment, or psychosis ("megaloblastic madness") can occur.
3. Screening
Routine screening is NOT recommended in the general population. Screening should be offered only when a patient has ≥1 clinical feature AND ≥1 risk factor for B12 deficiency.
| Society | Recommendation |
|---|
| ADA | Periodic B12 testing in patients on long-term metformin |
| AGA | Screen for B12 deficiency in patients with ileal resection or Crohn disease with extensive ileal disease |
| Long-term PPI use | No guideline currently recommends routine B12 testing in patients on long-term PPI, despite PPI being a listed risk factor. |
High-yield trap PPIs are a listed risk factor, but there is no guideline recommending routine screening for PPI users — a classic distractor on boards (don't confuse "risk factor" with "screen for it").
4. Diagnosis Algorithm
Step 1 — Initial test: Total serum vitamin B12
- Deficient if B12 < 180 pg/mL
- Borderline if B12 180–350 pg/mL → obtain methylmalonic acid (MMA)
Step 2 — Borderline B12 (180–350): Check MMA.
- MMA high → confirms true B12 deficiency
⚠ MMA may be falsely elevated in patients with kidney disease, dehydration, or thyroid disease — don't over-interpret MMA in these settings.
Step 3 — No clear cause identified?
Patients without an obvious explanation for their B12 deficiency (e.g., not vegan, no bariatric surgery) should be evaluated for atrophic gastritis (see Section 7).
5. Treatment & Dosing
Route
Oral or intramuscular supplementation both work. IM may be more effective in severe deficiency.
Effective dose
1000 mcg of B12 is an effective dose (oral or IM).
Testable Oral supplementation is as effective as intramuscular specifically in patients with pernicious anemia and post Roux-en-Y bypass — a common "trick" question (people assume malabsorption always needs IM).
Shared Decision-Making Treatment Pathway
Vitamin B12 deficiency in an outpatient
(refer to separate algorithm for diagnostic testing)
↓
Are any of the following concerning features present?
Symptomatic/severe anemia (Hb <8 g/dL) · Neuropsychiatric or neurologic symptoms · Possible/likely malabsorption · Concern about medication nonadherence
Initial parenteral (intramuscular) therapy
- Dose: During the first week, 1000 mcg IM 1 to 3 times weekly; then 1000 mcg IM once per week for up to 4 weeks
- Duration: Typically 4 weeks; if neuropsychiatric/neurologic symptoms are improving this may be continued up to 3 months
- Monitor: Close follow-up for resolution of symptoms, improvement in anemia and/or macrocytosis. Frequency depends on level of concern & severity. Repeat CBC at 2–4 weeks (may initially worsen)
Oral or IM therapy (shared decision-making)
- Dose: IM 1000 mcg weekly x4, then IM (in the U.S.) or hydroxocobalamin (once every 2 months); or Oral/SL: 1000–2000 mcg daily (available OTC)
- Monitor: Response expected? → if Yes, move to Maintenance therapy. If No, Re-evaluate.
Maintenance therapy
- Dose: IM 1000 mcg (cyanocobalamin) once every 2 months, OR hydroxocobalamin, administered by clinician or the patient — or Oral/SL 1000–2000 mcg daily (option if symptoms have resolved and adherence assured)
- Duration: Indefinite for irreversible causes of deficiency; if reversible cause addressed (e.g., metformin discontinued), therapy may be stopped
- Monitor: CBC every 6 months for the first year, then annually if immunity is confirmed and concerns for recurrence are low; routine B12/MMA monitoring is not required but may be individualized
Re-evaluate (if not responding)
- Consider other causes of anemia in addition to B12 deficiency (iron deficiency, folate deficiency, others)
- Consider other causes of neuropsychiatric symptoms
- Need for more intensive dosing if B12 levels remain borderline/low
- Continue therapy — neuropsychiatric/neurologic symptoms may take longer to improve and recovery may be incomplete
6. High Vitamin B12 Level — NOT Just "Extra Is Fine"
Definition: B12 > 1000 pg/mL (738 pmol/L) on two separate measurements.
Testable High B12 is associated with an increased risk of malignancy — solid tumors and hematologic malignancy (chronic myeloid leukemia, polycythemia vera, myeloproliferative syndromes) — as well as chronic liver and kidney disease, cardiovascular mortality, and autoimmune disease.
Proposed work-up
- Appropriate age-based cancer screening
- Labs for hematologic, liver, and kidney disease
Some experts recommend
- Complete blood count (CBC)
- Comprehensive metabolic panel (CMP)
- Liver ultrasonography
7. Atrophic Gastritis / Pernicious Anemia
1) Symptoms
- Dyspepsia
- Early satiety
- Bloating
- Features of B12 deficiency
2) When to suspect
- Unexplained vitamin B12 deficiency
- Family history of atrophic gastritis
- Personal history of an autoimmune condition
3) Diagnosis
- H. pylori testing
- Anti–intrinsic factor antibody → highly specific
- Anti–parietal cell antibody → highly sensitive
- Definitive diagnosis: endoscopic biopsy if antibody positive, or if pernicious anemia is still suspected despite negative antibodies
4) Complications
- Gastric cancer
- Micronutrient deficiencies
- → Requires routine surveillance endoscopy
Board pearl Memory trick: "Specific = Intrinsic factor Ab" and "Sensitive = Stomach parietal cell Ab." Anti-intrinsic factor antibody rarely gives false positives (specific) but misses many cases (low sensitivity); anti-parietal cell antibody catches most cases (sensitive) but is less specific.
8. Vitamin B12 vs Folate Deficiency
| Vitamin B12 | Folate |
|---|
| Diet | More in strict vegan or vegetarian diets | More in markedly decreased dietary intake or excessive alcohol intake |
| Symptoms | Subacute combined degeneration of the cord (progressive weakness, ataxia, paresthesias → can advance to spastic paraplegia) | Can still present with neurological symptoms (but classic cord degeneration is a B12 feature) |
| Time course | Usually occurs over years | Can occur in weeks to months |
Must-know rule Do NOT treat folate deficiency before checking B12. Giving folate alone can correct the megaloblastic anemia and mask an underlying B12 deficiency while its neurologic damage silently progresses (and may become irreversible).
9. Rapid-Fire Exam Pearls
Roux-en-Y = highest bariatric-surgery risk for B12 deficiency, yet oral supplementation is still as effective as IM in these patients.
Pregnancy ↑ risk of B12 deficiency, but routine prenatal B12 supplementation is not recommended.
Metformin + ADA → periodic B12 testing recommended for long-term use.
Crohn's/ileal resection + AGA → screen for B12 deficiency.
PPI is a risk factor but no guideline currently recommends routine screening on it.
B12 <180 = deficient; 180–350 = borderline → check MMA; MMA falsely ↑ in kidney disease, dehydration, thyroid disease.
Unexplained B12 deficiency → work up for atrophic gastritis (intrinsic factor Ab = specific; parietal cell Ab = sensitive; biopsy = definitive).
B12 >1000 pg/mL x2 → not reassuring — screen for malignancy (esp. CML, polycythemia vera), liver/kidney/CV disease.
Effective replacement dose = 1000 mcg (oral or IM); indefinite therapy if the underlying cause is irreversible.
Never treat folate deficiency without first ruling out/treating B12 deficiency — risk of masking/worsening neurologic disease.