Methylcobalamin is the active form of vitamin B12, directly usable by the body without further processing.
Conversely, cyanocobalamin is an inert synthetic form of B12, which must first be converted in the liver into methylcobalamin or adenosylcobalamin, the two biologically active forms.
This conversion process is not always optimal: it may involve partial loss of the vitamin, mobilize cellular energy, and be slowed down in certain individuals (notably in cases of oxidative stress, liver disorders or genetic polymorphisms). In addition, cyanocobalamin releases a cyanide molecule during conversion, which the body must then eliminate.
Although the names are similar, cyanocobalamin (synthetic form) and methylcobalamin (natural active form) do not have the same efficacy or bioavailability.
Some dietary supplements are simply labelled “cobalamin”. This is an indirect way of saying that they use cyanocolamamine, the inactive form of vitamin B12.