What are the benefits of a b12 vitamin and can i overdose on it?

Yes you can overdose: On any vitamin, you only need it if you are deficient in it as shown on a blood test, .

B12: B12 is involved with metabolism and cell replication. If you are low (you don't eat any meat (including fish) or you have a disease that prevents absorption then a supplement can help. Usually you will urinate out any excess of what your bodies needs. Like everything it can be harmful in excess, so take it only if you need it. You can get a blood test to find if you are low.