While there aren't very many side effects of too much vitamin B12 to be concerned with, there are a few things you should be on the lookout for if you are supplementing vitamin B12.
In rare cases, excess B12 supplementation may cause some numbness or tingling in the arm, hands and face.
According to the University of Maryland, people with Leber's Disease should not supplement vitamin B12, because it can cause damage to the optic nerve.
There appears to be a link between megadoses of vitamin B12 and certain cancers. One study showed that excess vitamin B12 intake was associated with a three-fold increase in risk of developing prostate cancer.