Pre-activated methyl-carrying form used directly by methionine synthase (remethylation of homocysteine → methionine (see methionine sam cycle)). Does not cover mitochondrial methylmalonyl-CoA mutase — that requires adenosylcobalamin, which cells synthesise from reduced cobalamins. Preferred by some authors in B12 deficiency with neurologic features, though head-to-head clinical superiority over cyanocobalamin is not established.