Dual active B12 forms that bypass MTR/MTRR-dependent conversion. Methylcobalamin is the active, neurologically bioavailable form that directly supports homocysteine recycling and neurological function. Hydroxocobalamin has a longer circulating half-life and converts to active B12 as needed. Together, they maintain holotranscobalamin, the bioavailable B12 fraction. Cyanocobalamin (the standard form) releases a cyanide metabolite and requires conversion steps that may be impaired in MTR/MTRR variants.