There is no upper limit for B12. The lab ranges often end around 800-900. But there is no recorded high being harmful anywhere in the literature for over 40 yrs! The FDA has not set an upper limit on B12 therefore. Most of us at PN keep ours above 1000. Dr. Snow who in 1999 changed B12 interventions and therapy has in his article, reasons for using B12 even if you are normal. He found relatively normal patients with neuro problems who improved with B12 supplements anyway irregardless of test results in the 400's or so.

There is NO evidence that high levels are dangerous.
Toxicity

from http://lpi.oregonstate.edu/infocente...ns/vitaminB12/

In fact lab ranges below 400 are still considered "normal" and are NOT. This is where the real problems reside.

My thread has medical sites for doctors on it early on, and also Dr. Snow's paper on post #70 later on. B12 is very misunderstood, and that need correcting for many patients today, esp if you are over 50 and even healthy!
People using drugs, acid lowering drugs, and some others, will develop B12 loss, and not even know it.