Answer
Codeine must be converted into morphine by CYP2D6. Genetic differences mean some people form little morphine and obtain limited benefit; persistent or worsening pain needs clinical review rather than self-escalation.
Sources for this answer
- UK GovernmentUK Government · Reputable Web
- Co-codamol 8/500 Tablets — Current selected SmPCemc · Summary of Product Characteristics
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.