Precursor to metabolite: Hydrocodone ER (q24h) to Hydromorphone

Hydrocodone ER (q24h) is converted to Hydromorphone by CYP2D6.

The pair

Hydrocodone ER (q24h) — Once-daily extended-release hydrocodone for continuous severe pain; abuse-deterrent tablet.
Hydromorphone — Potent semisynthetic opioid several times stronger than morphine.

Anything that inhibits or induces CYP2D6 shifts the balance between Hydrocodone ER (q24h) and Hydromorphone.