Think of it like this also in terms of bio availbility and the prevention of inactivity due to various complex mechanisms.....
Cyanocobalamin is the least friendly analogue of B12. Conversion equation.. cyanocoblamin>methylcobalamin>adenosylcobalamin Everytime that metabolic conversion occured there was a percentage of the original threshold administered lost to inactivity/malabsorption/degredation.
Hydroxocobalamin is similar to cyanocobalamin in the conversion process (hydroxocobalamin>methylcobalamin>adenosylcobalamin) It is generally more positive than cyano due to its protein complex and the extended half life. Cyanocobalamin and hydroxocobalamin are both inactive until the enzymatic alteration occur.
Methylcobalamin is considered biologically active and is immediately working upon ingestion. However further down the line and a few chemical reactions later it contributes methionine which even fuuuurther down the line it contributes a methyl group to the equation and this leads to adensoyl. This leads you to adensoylcobalamin.
Adensoylcobalamin is a biologically active form and will no longer undergo any ezymatic processes associated with further structural alterations. It is the key contributor in the metabolic pathways associated with ATP production.
Food for thought if your like me and love to read anything slightly educational.....
http://www.efsa.europa.eu/en/scdocs/doc/ans_ej815_vitamin_B12_op_en,0.pdf?ssbinary=true
So picking a analogue that has a more simplistic pharmacokinetics would be the smartest decision in intravenous administration of B12.
Also don't think of B12 as an esterized hormone or something that will inevitably reach its useless mark. The initial spike and subsequent stable serums are only one component. Your body stores B12 as well. The average adult can roughly hold 2mg-5mg of B12. At the most minimalist point of view it could theoretically provide a few years abet the last little bit isn't going to be pleasant!