Vitamin B12 is water-soluble, which means that the body will only use what it NEEDS from food/supplements and the rest...well you'll pee it out basically. There is really no major risk about taking "high" levels of water-soluble vitamins (unless you have some kind of medical problem related to them that could potentially interfere with absorption/excretion). Also, because of its nature as water-soluble, you have to continuously supply it in your diet (or via supplement). Your body can't store it up for future use (only the few fat-soluble vitamins are capable of being stored- Vit A, D, E and K).
However, I wouldn't recommend taking a supplement that has more than the 100% DV (or RDA?) you need each day. What that is, I don't know off the top of my head, but your body can only use so much at a time--the rest is wasted (and money down the drain).
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