If you have read a little about vegan food and nutrition, you know that vegans often have lower B12 levels than others. It's important to make sure your B12 levels don't get too low. Too little B12 is definitely not a good thing.
Normally, it is considered safe to take a lot of B12, but this is being discussed, and several seriously bad health conditions are associated with high B12 levels.
Here are some excerpts of what others have written about taking too much B12:
"What is the risk of overdose? Patients of Dr. H.L Newbold in New York City injected themselves three times daily with triple-strength doses of B12 (9,000 ug per day) indefinitely. Their serum B12 levels reached 200,000 pg/ml and more, but none had any significant side effects. Dr. Dommisse prefers hydroxycobalamin to cyanocobalamin. A person taking the latter might over a period of years accumulate a toxic amount of cyanide and possibly damage vision. Methylcobalamin, according to one source, might be risky for schizophrenic patients." http://www.newmediaexplorer.org/chri...er%20final.rtf
Another one: "Brewer's yeast fortified with B12 is a good source of this nutrient. However, be aware that prolonged and excessive use of brewer's yeast can impair kidney function." http://www.innvista.com/health/nutri...tamins/b12.htm