Vitamin B12 (Cobalamin) Deficiency — Treatment Guidelines

1. Confirm the Diagnosis First

• Serum cobalamin (B12): levels <74 pmol/L (100 ng/L) are clearly deficient; 74–148 pmol/L is borderline
• Serum methylmalonic acid (MMA): elevated in true deficiency (>0.75 µmol/L is confirmatory); useful when serum B12 is borderline (125–250 pmol/L)
• Plasma homocysteine: elevated in both B12 and folate deficiency (less specific)
• Note: anti-intrinsic factor antibodies can cause false-normal serum B12 in up to 50% of pernicious anemia (PA) cases — if clinical suspicion is high, measure MMA

2. Choose Route Based on Cause

3. Dosing Regimens

IM/SC (Cyanocobalamin or Hydroxocobalamin)

• Hydroxocobalamin (preferred in UK): 1,000 µg IM every other day for 2 weeks → then every 3 months for life
• Cyanocobalamin (preferred in US): 100 µg/day IM × 6–7 days; if improvement, 100 µg every 3–4 days × 2–3 weeks; maintenance: 1,000 µg IM monthly
• For maintenance, hydroxocobalamin is retained better than cyanocobalamin, so monthly dosing suffices vs. the more frequent cyanocobalamin protocols

• Begin injections immediately; continue every other day until no further improvement, then every 3 months
• Greatest recovery occurs when symptoms have been present <3 months; progression is arrested even in longer-standing disease
• Adams & Victor's Neurology 12th ed notes: "at least 50% of patients exhibit some permanent neurologic deficit" if treatment is delayed

• Deficiency: 100 µg/day × 10–15 days, then 100 µg once or twice weekly × several months → maintenance: ≥60 µg/month
• Pernicious anemia: 30–50 µg/day for ≥14 days (total 1,000–5,000 µg) → maintenance: 100 µg/month

Oral Therapy

Intranasal / Sublingual (Maintenance options)

• Intranasal cyanocobalamin gel: 500 µg in one nostril once weekly (maintenance)
• Sublingual: 1,000–2,000 µg/day (maintenance option)

4. NICE NG239 (2024) Key Recommendations

1. Dietary deficiency → oral B12 ± dietary advice; consider IM if oral is not feasible
2. Malabsorption (non-autoimmune) → offer B12 replacement; consider IM over oral
3. Pernicious anemia / autoimmune gastritis / total gastrectomy / complete ileal resection → IM injections, lifelong
4. If symptoms not improving on oral → increase to maximum licensed oral dose, or switch to IM
5. Neurological symptoms → IM preferred to ensure rapid and reliable delivery
6. Follow-up: Reassess symptoms; use serum MMA or homocysteine to confirm resolution

5. Special Situations

6. Response to Treatment

• Hematological response: Reticulocytosis peaks at 5–7 days; hemoglobin normalizes in 6–8 weeks
• Neurological response: Improvement begins in first 3–6 months; slower thereafter; recovery depends on symptom duration
  • < 3 months of neurological symptoms → usually complete recovery
  • Longer duration → partial improvement or arrested progression
• Biochemical monitoring: MMA and homocysteine normalize with effective treatment

Summary: Route Selection at a Glance

Cause identified?
├── Dietary only → ORAL (≥500 µg/day)
├── Food-cobalamin malabsorption (achlorhydria, aging) → ORAL (≥1,000 µg/day)
├── Pernicious anemia / autoimmune gastritis → IM LIFELONG
├── Total gastrectomy / complete ileal resection → IM LIFELONG
├── Partial gastrectomy / coeliac / bariatric → ORAL or IM (prefer IM)
├── Neurological involvement → IM (prioritize)
└── Unknown cause / treatment failure → escalate to IM

• Harrison's Principles of Internal Medicine, 22nd ed. (2025)
• Yamada's Textbook of Gastroenterology, 7th ed.
• Adams & Victor's Principles of Neurology, 12th ed.
• Harriet Lane Handbook, 23rd ed.
• NICE Guideline NG239: Vitamin B12 deficiency in over 16s (March 2024)
• Abdelwahab OA et al. Irish J Med Sci 2024 — PMID 38231320: All routes (IM, oral, sublingual) are equivalently effective in raising B12 levels

Vitamin B12 (Cobalamin) Deficiency — Treatment Guidelines

1. Confirm the Diagnosis First

2. Route of Administration — Determined by Cause

3. Dosing Regimens

IM / SC

• Loading: 1,000 µg IM on alternate days for 2 weeks
• Maintenance (no neurological symptoms): 1,000 µg IM every 3 months
• Maintenance (neurological symptoms): Alternate-day injections until no further improvement → then every 2–3 months

• Loading: 1,000 µg IM daily × 7 days → weekly × 4 weeks
• Maintenance: 1,000 µg IM monthly
• (Cyanocobalamin is retained less well than hydroxocobalamin, hence more frequent dosing)

• Loading: 100 µg/day IM × 10–15 days
• Maintenance: ≥60 µg/month (deficiency) or 100 µg/month (pernicious anemia)

Oral Therapy (High-Dose Passive Absorption)

Intranasal / Sublingual (Maintenance)

• Intranasal cyanocobalamin: 500 µg in one nostril once weekly
• Sublingual: 1,000–2,000 µg/day

4. NICE NG239 (March 2024) Algorithm

What is the cause?
│
├── Dietary → Oral B12 + dietary advice
│              (consider IM if oral not tolerated)
│
├── Food-cobalamin malabsorption → Oral ≥1 mg/day
│
├── Pernicious anemia / autoimmune gastritis
│   / total gastrectomy / complete ileal resection → IM LIFELONG
│
├── Partial gastrectomy / coeliac / bariatric → IM preferred over oral
│
└── Unknown / treatment failure on oral
    → Escalate oral to maximum licensed dose
      OR switch to IM

5. Special Considerations

6. Expected Response to Treatment

• Harrison's Principles of Internal Medicine, 22nd ed. (2025)
• Yamada's Textbook of Gastroenterology, 7th ed.
• Adams & Victor's Principles of Neurology, 12th ed.
• Harriet Lane Handbook, 23rd ed.
• NICE NG239 — Vitamin B12 deficiency in over 16s (March 2024)
• Abdelwahab OA et al., Irish J Med Sci 2024 PMID 38231320