Vitamin B12 over 300 pg/mL

Above 300 pg/mL you are past the ODS marginal band, and above 350 you are in the range NICE labels "Test result suggests vitamin B12 deficiency is unlikely." NICE's instruction for that result is to look elsewhere: "investigate other causes of their symptoms and if they are still experiencing symptoms 3 to 6 months later, consider a repeat of the initial test." No body sets an upper reference value or an optimal level. ODS defines no optimum; NICE's table stops at "unlikely."

Why a high number can mislead. NICE recommendation 1.3.8 is the sentence to know: an over-the-counter B12 product, whether tablets, injections or patches, "may increase total or active B12 concentrations without fully treating a deficiency." So a result taken while supplementing does not close the question the way an unsupplemented one would, and NICE tells clinicians to ask about supplements before the first test. In the other direction, NICE says people of Black ethnicity may have a higher reference range than people of White or Asian ethnicity, so "people of Black ethnicity may need treatment even if their blood test results are above one of the cut-offs." The superseded 2014 BSH guideline made the same point about the number in general: a serum B12 above 200 ng/l "in the presence of a strong clinical suspicion of cobalamin deficiency should be evaluated further with MMA, tHcy or HoloTC and a trial of hydroxocobalamin."

Higher is not better. ODS: "vitamin B12 supplementation appears to have no beneficial effect on performance in the absence of a nutritional deficit," and randomized trials found B12 supplements lower homocysteine "but not CVD risk"; a Cochrane review of 15 studies in 71,422 people concluded they "do not prevent heart attacks or reduce death rates." Trials of 1 to 2 years did not improve cognition in older adults with or without dementia. On cancer ODS hedges both ways, reporting one analysis of 757,185 people in which a level above 813 pg/mL carried a higher 1-year cancer risk, and other evidence showing no link, with the conclusion that "More research is needed." There is no tolerable upper intake level because the vitamin has "low potential for toxicity," which is a statement about safety, not benefit.

What to do. If you take a B12 supplement and feel well, nothing. If you have symptoms from NICE's list, fatigue, pins and needles, balance problems, memory difficulty, glossitis, the result does not end the search, and NICE's path is other causes first, then a repeat test in 3 to 6 months if symptoms persist. ODS's list of who runs short is worth checking against your own situation even with a normal number: metformin, acid suppressants, prior stomach or intestinal surgery, and a diet without animal foods.