Isozyme morphine

As discussed above, the analgesic effect of codeine depends greatly on its abihty to be metabolized to morphine. In patients in whom large amounts of codeine have been administered without relief, the possibility of a polymorphism of the CYP2D6 enzyme must be considered. Medications that inhibit CYP2D6 may reduce or even completely block the conversion of codeine to morphine. These include selective serotonin reuptake inhibitors (SSRls), cimetidine, and antidepressants such as buproprion. On the other hand agents that induce CYP450 isozymes, such as rifampin and dexamethasone, increase the conversion rate and increase the risk of adverse events. 

---