You may have an allergy to hapten, but with the skin allergy, that is likely to be cobalt. (Cobalt is a component of B12 and causes skin rashes). However, the body naturally contains B12, so it seems that you can cope with small amounts of B12, but these tablets exceed the RDA (recommended daily amount) so maybe that is why you react. You need to speak to a doctor about this or a pharmacist!