Precursor to metabolite: Hydrocodone ER (q12h) to Hydromorphone

Hydrocodone ER (q12h) is converted to Hydromorphone by CYP2D6.

The pair

Hydrocodone ER (q12h) — Every-12-hour extended-release hydrocodone for severe persistent pain; not for as-needed use, with overdose and dependence risks.
Hydromorphone — Potent semisynthetic opioid several times stronger than morphine.

Anything that inhibits or induces CYP2D6 shifts the balance between Hydrocodone ER (q12h) and Hydromorphone.