What would happen if you didn't have enough vitamin B12?

There is no credentialed medical research that shows any medical consequences of Vitamin B12 overdose. B12 is a non-toxic substance. The liver stores a 3-5 year supply of B12, and the rest is sent directly to the colon for elimination. You are far more likely to have some kind of reaction to the inactive ingredients in your B12 tablets than you are to the B12 itself.