Answer
CYP2D6 converts codeine into morphine. People with low enzyme activity may obtain little benefit, while ultra-rapid metabolisers can produce excessive morphine and develop serious toxicity.
Sources for this answer
- Codeine phosphate 30mg Tablets — Current selected SmPCemc · Summary of Product Characteristics
- UK GovernmentUK Government · Reputable Web
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.