In a 72-year-old man taking daily sublingual methylcobalamin with a serum vitamin B12 of 834 pg/mL, should I discontinue the supplement and recheck his B12 level?

Should You Discontinue Sublingual B12 Methylcobalamin in a 72-Year-Old with B12 Level of 834 pg/mL?

No, do not discontinue the sublingual methylcobalamin supplement in this patient. A serum B12 of 834 pg/mL is well above the normal range but does not indicate toxicity or harm, and stopping supplementation without understanding the underlying reason for use may lead to recurrent deficiency. 1

Why Elevated B12 Levels Are Not Harmful from Supplementation

• Vitamin B12 has no established upper tolerable limit because excess amounts are readily excreted in urine without toxicity—there are no documented cases of acute toxicity from B12 supplementation, even at very high doses. 1
• The elevated level of 834 pg/mL is iatrogenic (supplement-induced) and represents recent oral intake rather than a pathological state requiring intervention. 2
• Sublingual methylcobalamin is an effective and safe route of B12 administration, with studies demonstrating normalization of B12 levels and metabolic markers (methylmalonic acid, homocysteine) in deficient patients using doses of 350-2000 μg weekly. 3, 4, 5

When Elevated B12 Levels Do Require Investigation

However, you must distinguish supplement-induced elevation from pathological causes. Serum B12 >350 pg/mL can be a warning sign of serious underlying disease when it occurs without supplementation. 2, 6

Pathological causes of elevated B12 include:

• Hematologic malignancies (myeloproliferative disorders, particularly those with eosinophilia and PDGFRA fusion genes) 2
• Solid organ malignancies 6
• Liver disease (cirrhosis, acute hepatitis, hepatocellular carcinoma, metastatic disease) causing release of stored B12 from damaged hepatocytes 2, 6
• Renal disease 6
• Critical illness (highest levels observed in non-survivors, functioning as a negative prognostic marker) 2

If B12 is elevated without supplementation, perform:

• Complete blood count with differential (assess for eosinophilia, dysplasia, monocytosis, blasts) 2
• Comprehensive metabolic panel with liver function tests 2
• Serum tryptase (often elevated in myeloproliferative disorders) 2
• Peripheral blood smear review 2

Critical Questions to Answer Before Making Any Changes

1. Why was the patient started on B12 supplementation?

You must determine the original indication:

• Documented B12 deficiency (prior level <180 pg/mL or 180-350 pg/mL with elevated methylmalonic acid) 1

• Malabsorption conditions requiring lifelong supplementation:

  • Pernicious anemia (anti-intrinsic factor antibodies positive) 7
  • Post-bariatric surgery (Roux-en-Y, sleeve gastrectomy) 7, 1
  • Crohn's disease with ileal involvement >20 cm 7, 1
  • Total or partial gastrectomy 7
  • Atrophic gastritis 7
  • Chronic PPI or H2-blocker use >12 months 1
  • Metformin use >4 months 1

• If the patient has any of these conditions, B12 supplementation must continue for life, regardless of serum levels, because the underlying malabsorption is permanent. 7, 1

2. Does the patient have symptoms of B12 deficiency?

Even with a "normal" serum B12, functional deficiency can occur at any serum level due to impaired tissue uptake. 6 Assess for:

• Neurological symptoms: paresthesias, numbness, cognitive difficulties, memory problems, gait disturbance, peripheral neuropathy 1
• Hematologic findings: macrocytosis (MCV >98 fL), anemia 1
• Oral symptoms: glossitis, glossodynia, fissured tongue 1
• Fatigue or mood disturbances 1

If symptoms persist despite elevated serum B12, measure methylmalonic acid (MMA) to assess functional B12 status—up to 50% of patients with "normal" serum B12 have metabolic deficiency when MMA is measured. 1

3. Is the patient at high risk for deficiency recurrence?

• Age >75 years: 18.1% of adults >80 years have metabolic B12 deficiency despite normal serum levels 1
• Autoimmune thyroid disease: 28-68% prevalence of B12 deficiency, requiring annual screening 1
• Post-stroke or cardiovascular disease: 17.3% have biochemical or metabolic deficiency 1
• Vegetarian/vegan diet: limited dietary B12 intake 1

Recommended Clinical Algorithm

Step 1: Confirm the patient is taking sublingual B12

• Verify dose, frequency, and formulation (methylcobalamin vs cyanocobalamin)
• Confirm timing of last dose relative to blood draw (recent supplementation explains elevation) 2

Step 2: Review the original indication for supplementation

• If documented malabsorption condition (pernicious anemia, bariatric surgery, ileal resection >20 cm, atrophic gastritis): Continue supplementation indefinitely 7, 1
• If prior documented deficiency without identified cause: Consider trial discontinuation with close monitoring (see Step 4)

Step 3: Assess for symptoms of functional deficiency

• If symptomatic despite elevated serum B12: Measure MMA (>271 nmol/L confirms functional deficiency) 1
• If MMA elevated: Continue or increase B12 supplementation regardless of serum level 1

Step 4: If considering discontinuation (only in patients without malabsorption and no symptoms)

• Do not measure serum B12 immediately after stopping—it reflects recent supplementation, not functional status 1
• Recheck B12 level in 3-6 months after discontinuation to allow washout 1
• If B12 falls to 180-350 pg/mL, measure MMA to confirm functional status before restarting 1
• If B12 falls to <180 pg/mL, restart supplementation immediately 1

Step 5: Optimize supplementation regimen if continuing

For patients with malabsorption requiring lifelong therapy:

• Oral route: 1000-2000 μg daily (as effective as intramuscular for most patients, including those with malabsorption) 1, 8
• Sublingual route: 350 μg weekly (50 μg/day) or 2000 μg weekly (both equally effective) 3
• Intramuscular route: 1000 μg monthly (reserved for severe neurological symptoms, confirmed malabsorption, or oral therapy failure) 7, 1

Methylcobalamin or hydroxocobalamin are preferred over cyanocobalamin, especially in patients with renal dysfunction or cardiovascular disease. 1, 9

Common Pitfalls to Avoid

• Do not stop B12 in patients with permanent malabsorption conditions (pernicious anemia, bariatric surgery, ileal resection >20 cm)—these patients require lifelong supplementation regardless of serum levels. 7, 1
• Do not rely solely on serum B12 to rule out deficiency—up to 50% of patients with "normal" serum B12 have metabolic deficiency when MMA is measured. 1
• Do not administer folic acid before treating B12 deficiency—folate can mask anemia while allowing irreversible neurological damage to progress. 1
• Do not measure serum B12 immediately after stopping supplementation—it reflects recent intake, not functional status; wait 3-6 months. 1
• Do not confuse supplement-induced elevation with pathological causes—if the patient is not taking supplements and B12 is >350 pg/mL, investigate for malignancy, liver disease, or critical illness. 2, 6

Monitoring Plan if Continuing Supplementation

• Recheck B12 at 3-6 months to confirm stable levels on current regimen 1
• Annual B12 screening for high-risk populations (age >75, autoimmune thyroid disease, post-bariatric surgery, metformin use) 1
• Check concurrent micronutrients (folate, iron, vitamin D, thiamine) at the same intervals, as deficiencies often coexist 1
• Monitor MCV and complete blood count to detect macrocytosis or anemia 1
• Target homocysteine <10 μmol/L for optimal cardiovascular outcomes in patients with cardiovascular disease or stroke history 1