There is no upper limit for B12, because nobody has looked
About 5 percent of a large oral dose is absorbed, and no tolerable upper intake has been set, for the general population or for pregnancy. Harm has not been demonstrated.
What the study found
Only about 5 percent of a large oral B12 dose is absorbed
Which is part of why no tolerable upper intake has been set for vitamin B12, for the general population or for pregnancy: harm at high doses has not been demonstrated. That is an absence of evidence, not a clean bill of health.
The caveat · Figures cited second-hand, from 2012
Both figures are background citations inside a review about pregnancy trials rather than results its authors produced, so they carry the weight of a textbook footnote and not of a measurement. The review dates from 2012 and predates the later cohort reports on high blood B12 and cancer.
What we don't know
Both figures are background citations inside a review about pregnancy trials rather than results its authors produced, so they carry the weight of a textbook footnote and not of a measurement. The review dates from 2012 and predates the later cohort reports on high blood B12 and cancer.