Re: B-12 is there such a thing as too much?
From the U.S. Institute of Medicine, the latest (1998) DRI guidelines for vitamin B12 state that there isn't enough evidence of toxicity from excessive B12 intake from foods or supplements. The report goes on to explain why there are no upper limits for B12, which include the GI tract only being able to absorb a small percentage when large doses are ingested, and the weak evidence that although animal studies showed the opposite, excessive B12 intake in humans showed that tumour formation in the colon and liver is inhibited.
Current RDI for adults is 2.4 mcg, though up to 100 mcg a day have been reported with no toxic effects. But even so, chronically excessive amounts of anything should be avoided. Is there a reason why your B12 levels are high? If you take in a lot of dietary B12 and you're worried about adverse effects, you can try to cut down on B12 sources.