Originally Posted by mrsD

400pg/ml is the new suggested low today. Unfortunately labs still go down to 200...and do not reflect the new information about B12 treatments.

This link is to a decade+ old article which is a physician continuing education about low B12. The 400pg/ml suggestion appears on it along with an explanation that oral works...
Vitamin B12 Deficiency - American Family Physician

This is a link to our B12 sticky thread ...it goes further and explains the DNA methylation errors and other factors about B12 including using oral on an empty stomach, etc.
STICKY - The Vitamin B12 Thread:

Most labs do not go above 2000 in reporting. This is because of their calibration techniques. Most of the posters here keep their B12 levels at 1000 or above. National health recommendations suggest people in the US at age 50 or above take a B12 supplement whether they have medical symptoms or not. The best option is methylcobalamin because new research shows genetic failures up to 40% or more, which prevents cobalamin methylation in the body which is the activation to active methylcobalamin.

This methylation information can be found on MTHFR.net.

B12 is very safe and has no upper limit assigned to it by
the FDA. The only contraindication I have found is this...
if a person is diagnosed with polycythemia vera (too many red blood cells) they should avoid supplements of folate and B12 use without a doctor's supervision.

If you take a B12 supplement discontinue it about a week before any further blood testing, to avoid a false elevation result.