Precursor to metabolite: Codeine to Morphine

Codeine is converted to Morphine by CYP2D6.

The pair

Codeine — Weak opioid and prodrug; CYP2D6 variability makes response unpredictable.
Morphine — The prototypical opioid analgesic and comparison standard.

Anything that inhibits or induces CYP2D6 shifts the balance between Codeine and Morphine.