This is info I found (gotta love GOOGLE)
Can You Overdose on Vitamin B12?
Vitamin B
12 is a fairly non-toxic substance that is unlikely to cause problems in cases of an overdose. Even so, if you happen to overdose on vitamin B
12, it is a good idea to seek immediate medical attention.
Symptoms of a Vitamin B12 Overdose
There are no "symptoms" of a vitamin B
12 overdose, per se. Even very large doses do not seem to cause problems for most people. In rare cases, people may experience vitamin B
12 side effects
(see Vitamin B12 Toxicity), but these side effects are probably just as common with normal doses as with overdoses.
In the case of massive overdoses, toxic effects might occur due to the inactive ingredients in vitamin B
12 supplements. Sometimes vitamin B
12 comes in combination with other vitamins or other supplements. Of course, an overdose of such a product might cause overdose symptoms and effects due to the other components of the product.
Treating a Vitamin B12 Overdose
Treatment for an overdose of vitamin B
12 -- if necessary -- will involve supportive care, which consists of treating the symptoms that occur as a result of the overdose. Of course, treatment will also involve stopping the vitamin and then resuming it at a more appropriate dose, if necessary.
It is important that you seek prompt medical attention if you believe that you may have overdosed on vitamin B
12.