Precursor to metabolite: Tramadol to O-desmethyl- tramadol

Tramadol is converted to O-desmethyl- tramadol by CYP2D6.

The pair

Tramadol — Weak mu agonist plus SNRI; seizure and serotonin-syndrome risk, misuse potential.
O-desmethyl- tramadol — The active metabolite of tramadol; a much stronger mu agonist than the parent.

Anything that inhibits or induces CYP2D6 shifts the balance between Tramadol and O-desmethyl- tramadol.