"Some medications are CYP2D6 inhibitors and reduce or even completely block the conversion of codeine to morphine. The best-known of these are two of the
selective serotonin reuptake inhibitors,
paroxetine (Paxil) and
fluoxetine (Prozac) as well as the antihistamine
diphenhydramine (Benadryl) and the antidepressant
bupropion (Wellbutrin, also known as Zyban). Other drugs, such as
rifampicin and
dexamethasone, induce CYP450 isozymes and thus increase the conversion rate.
CYP2D6 converts codeine into morphine, which then undergoes glucuronidation. Life-threatening intoxication, including respiratory depression requiring intubation, can develop over a matter of days in patients who have multiple functional alleles of CYP2D6, resulting in ultrarapid metabolism of opioids such as codeine into morphine.
[38][39][40]"
wikipedia
Ratched, I think the logic is that if you have several metabolic routes of which some lead to inactive compounds and some active compounds, you get more bang if you block the routes leading to inactive compounds