Methylcobalamin (mecobalamin, MeCbl, or MeB12) is a cobalamin, a form of vitamin B12. It differs from cyanocobalamin in that the cyano at the cobalt is replaced with a methyl group. Methylcobalamin features an octahedral cobalt(III) centre and can be obtained as bright red crystals. From the perspective of coordination chemistry, methylcobalamin is notable as a rare example of a compound that contains metal-alkyl bonds. Nickel-methyl intermediates have been proposed for the final step of methanogenesis. Methylcobalamin is equivalent physiologically to vitamin B12,[citation needed] and can be used to prevent or treat pathology arising from a lack of vitamin B12 intake (vitamin B12 deficiency).
Methylcobalamin is also used in the treatment of peripheral neuropathy, diabetic neuropathy, and as a preliminary treatment for  amyotrophic lateral sclerosis. Methylcobalamin that is ingested is not used directly as a cofactor, but is first converted by MMACHC into cob(II) alamin. Cob(II)alamin is then later converted into the other 2 forms, adenosylcobalamin and methylcobalamin for use as cofactors. That is, methylcobalamin is first dealkylated and then regenerated. One author says it is important to treat vitamin B12 deficiency with hydroxocobalamin or cyanocobalamin or a combination of adenosylcobalamin and methylcobalamin, and not methylcobalamin alone.