The liver converts codeine into morphine using the enzyme CYP2D6, a process that causes pain relief in most people. However, genetic differences in some people cause either too much or too little ...

Promethazine W. Codeine Dosage and Administration Adult Use lowest effective dose for shortest duration. Use accurate measuring device. ≥12yrs: 5mL every 4–6hrs; max 30mL/24hrs.

It has been suggested that the primary analgesic response from codeine results from the 10% that is metabolized to morphine by CYP2D6. [29, 30] Therefore, patients with no or reduced CYP2D6 ...