How Long to Stop B12 Supplements Before Retesting
You should stop oral B12 supplements for at least 10-14 days before retesting your B12 levels to allow for clearance of exogenous supplementation and obtain an accurate assessment of your true B12 status.
Rationale for the Washout Period
While the guidelines do not provide explicit washout periods, the pharmacokinetics of oral B12 supplementation and clinical practice considerations support this timeframe:
- Oral B12 supplements can artificially elevate serum B12 levels for several days after discontinuation, potentially masking ongoing deficiency or malabsorption issues 1
- Standard serum B12 tests measure total circulating cobalamin, which includes both the biologically active form and recently absorbed supplemental B12 that may not reflect your body's actual functional B12 status 2
- A 10-14 day washout period allows most of the acutely absorbed supplemental B12 to clear from circulation while still maintaining safety, as B12 stores in the liver can last 2-3 years 2
Important Testing Considerations
Consider more sensitive biomarkers instead of or in addition to total serum B12:
- Methylmalonic acid (MMA) is a more sensitive marker of functional B12 status and should be measured if B12 levels are indeterminate or if clinical suspicion remains high despite normal B12 levels 2, 1
- Holotranscobalamin (active B12) measures the biologically active form available for cellular use and may be superior to total serum B12, though it's not routinely tested 2, 3
- Homocysteine levels can also indicate functional B12 deficiency, with target levels <10 μmol/L considered optimal 4
Critical Caveats
- Do not stop B12 supplementation if you have neurological symptoms (paresthesias, gait disturbances, cognitive changes, memory problems), as these can become irreversible if untreated 2, 5
- Standard serum B12 tests may not accurately reflect functional B12 status - up to 50% of patients with "normal" serum B12 have metabolic deficiency when measured by MMA 2
- If you have confirmed malabsorption (pernicious anemia, ileal resection, bariatric surgery), retesting serum B12 levels after stopping supplements is less clinically relevant than monitoring for symptom resolution and may not change management, as you'll likely require lifelong supplementation regardless 4, 5
When Retesting May Not Be Necessary
You may not need to retest at all if:
- You have confirmed intrinsic factor antibodies or pernicious anemia - lifelong treatment is necessary regardless of levels 2
- You've had >20 cm of distal ileum resected or bariatric surgery - prophylactic monthly B12 is recommended for life 4
- Your symptoms have completely resolved on supplementation - continue treatment and monitor clinically rather than stopping to retest 5