If taken intranasally???
10 % of the codeine you ingest is transformed by demethylation to morphine in the liver, not in the brain.
I cant remember what cytochrome does that. Cyp450 means nothing really, as its a class of enzymes, playing an important role to detoxify your organism and make non-polar substances more polar for excretion, mostly by urine.
Which makes me wonder about the potential pain relief you could get from intranasal codeine. I can see the antitussiv properties, but without demethylation and direct access to the brain, what fraction will be transformed into morphine???