One painkiller, different metabolic routes
Codeine is an opioid, but much of its pain relief comes only after the body converts some of it into morphine. That conversion happens mainly in the liver, where the enzyme CYP2D6 acts like a molecular tool that clips a chemical group from codeine. The resulting morphine can bind to opioid receptors, which dampen pain signals in the brain and spinal cord.
People inherit different versions of the gene that makes CYP2D6. Some versions produce little enzyme activity; others can lead to unusually high activity. As a result, two people taking the same amount of codeine may produce very different amounts of morphine.
Why the difference matters
Someone who converts codeine slowly may get less pain relief, because less morphine is formed. Someone who converts it very quickly can generate more morphine than expected. If morphine accumulates, it can cause severe opioid effects, including extreme sleepiness and dangerously slowed breathing.
CYP2D6 activity is not determined by genes alone. Some medicines can inhibit the enzyme, temporarily reducing how much codeine is converted. That can change the drug’s effect even for someone whose inherited enzyme activity would otherwise be typical. Other bodily factors also influence how a person responds, so genetic metabolism is one important part of the picture—not a complete prediction of an individual’s reaction.
The practical consequence
A standard dose is a standard amount of codeine, not a guarantee of a standard amount of morphine. That gap helps explain why codeine may relieve pain for one person yet have little effect—or cause serious harm—for another. Clinicians may consider a person’s medicines, response, and, in some situations, genetic information when choosing pain treatment.
