Precursor to metabolite: Bupropion SR (smoking cessation) to Hydroxybupropion

Bupropion SR (smoking cessation) is converted to Hydroxybupropion by CYP2B6.

The pair

Bupropion SR (smoking cessation) — Sustained-release bupropion for smoking cessation; nonnicotine aid with antidepressant activity.
Hydroxybupropion — Bupropion's principal active metabolite; largely noradrenergic and present at higher concentrations than the parent.

Anything that inhibits or induces CYP2B6 shifts the balance between Bupropion SR (smoking cessation) and Hydroxybupropion.