--that very high levels of B12 can cause true nerve parastheses (pins and needles), though often the energizing effect of sudden large influxes of B12 can seem that way.

There have been cases of peopel reacting to additive in B12 supplements, though. In my case, it was sorbitol. One has to be careful of what the B12 is combined with. (Once I figured it out and stopped taking a sorbitol-containing brand, the effects dissipated.)

Now large doses of B6 (pyridoxine) taken over a period of time, that can have neurtoxic effects in some susceptible people:

http://neuromuscular.wustl.edu/nothe...yridoxineintox

I currently take 1000mcg (1 mg) methylcobalamin each day, primarily as a nerve protectant, considering my idiopathic acute onset full-body burning neuropathy history, and my current cervical spine radiculopathy. My last B12test was 1864 (and I'm probably due for another one, but I imagine it would be somewhere around there again).

As far as I know, B12 has no known toxicity level, Unlike other B-vitamins, there is a substantial bodily ability to store extra, primarily in the liver, though after a certain point excess will be excreted. But there have been some studies, particularly in Japan, in which subjects were given up to 36,000mcg (36mg)/day without ill effects.