Absorbing vitamin B12 in your body is not easy. Until now.
Most multivitamin pills only contain 100-200 mcg of the cyanocobalamin form of B12. This must then be converted to methylcobalamin or adenosylcobalamin before it can be used by the body.

The actual absorption of B12 is a serious problem with oral supplements. Swallowing 500 mcg of Vitamin B12 can result in absorption of as little as 1.8 mcg, so most oral supplements do not provide an adequate daily intake!   More detail...