Four cobalamin forms and what each is for
Vitamin B12 circulates and is sold in several forms. Methylcobalamin and adenosylcobalamin are the two coenzyme forms the body actually uses: methylcobalamin supports the methylation cycle (converting homocysteine to methionine), while adenosylcobalamin works in the mitochondria in branched-chain amino acid and odd-chain fatty acid metabolism. Hydroxocobalamin is a natural form often used in injections because it is retained well. Cyanocobalamin is a stable synthetic form that the body converts into the active forms after releasing a small amount of cyanide, an amount considered negligible for most people.
The diagram lines these up because the choice between them is mostly about conversion and retention, not raw potency.
Does methylation status change the best choice
A popular claim is that people with methylation variants such as MTHFR should choose methylcobalamin. The body interconverts B12 forms, so for most people any form corrects a deficiency. The case for a particular form is strongest in specific situations, for example hydroxocobalamin in cyanide exposure or where injection retention matters, rather than as a universal rule.
Evidence that one oral form is clearly superior for the general population is weak. This is general educational information, not medical advice; B12 deficiency, which can cause irreversible nerve damage if prolonged, should be diagnosed and treated under a qualified professional.