(hī-drok'sē-kō-bal'ă-min) A chemical compound, also called vitamin B12a, which is the immediate precursor to cyanocobalamin (vitamin B12) in the body and that has also been investigated as an antidote in cyanide poisoning, although it is not currently approved for such use in the U.S. See also: amyl nitrite, sodium nitrite, sodium thiosulfate Synonym(s): hydroxocobalamin.