Anything fat soluble vitamins can be dangerous in high doses. As can some minerals, such as iron. Water soluble vitamins are less dangerous, but you still should not take too much at once, especially if you are on some medications or have some medical conditions.
From what I've seen, B-12 isn't dangerous in high doses, but you should still tell you doctor all of the supplements you take, just in case they interact with something. However, you also don't get any extra benefit from taking more than your body can handle. You'll just be making expensive urine.
