Possible but rare
In general, B12 is very safe and well-tolerated and there is little danger in taking high doses.
and rashes have been reported. It is advised to avoid in leber's disease, a rare hereditary eye
condition. More info on B12 at http://lpi.Oregonstate.Edu/infocenter/vitamins/vitaminb12/ and http://www.Nlm.Nih.Gov/medlineplus/druginfo/natural/926.Html#safety.