Precursor to metabolite: Bupropion SR to Hydroxybupropion

Bupropion SR is converted to Hydroxybupropion by CYP2B6.

The pair

Bupropion SR — Sustained-release NDRI; activating and sexually sparing, but seizure-limited.
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 and Hydroxybupropion.