Cobalamin may refer to several chemical forms of vitamin B12, depending on the upper axial ligand of the cobalt ion. These are:

• Cyanocobalamin (R = –CN)
• Hydroxocobalamin (R = –OH)
• Methylcobalamin (R = –CH3), an active form of vitamin B12
• Adenosylcobalamin (R = –Ado), also an active form of vitamin B12

Cobalamin is susceptible to acid. During its biosynthesis, complexes are formed which protect it. (source)