Question about the formation of norcodeine from morphine in man

Metabolism of morphine and codeine by guinea-pig liver microsomal preparations and by rats

“…The nor-codeine formed was significantly higher from rat urine than from the washed microsomal preparation but the codeine formed was of about the same order (5.3-6.3%). In view of the controversy which exists about whether codeine is a metabolite of morphine (2)(3)(4)(6)(7)(8) it must be emphasised that the morphine used in this investigation contained no codeine (see Materials and Methods). Hence, with both the washed microsomal preparations and the rat in vivo experiments, codeine was definitely established as a metabolite of morphine.…”

“…Codeine and nor-codeine have been demonstrated as minor metabolites of morphine in rats and in man, using in vitro and in vivo techniques (3,4,5). These findings have been questioned because codeine may be present as an impurity in commercial samples of morphine (6,7) but this suggestion has been refuted (8). Morphine N-oxide has been implicated as a metabolite of morphine although it was not character-:t: Presented as a short communication to the 2nd International Conference on the Biological Oxidation of Nitrogen, held at Chelsea College, University of London, September 1977.…”