Precursor to metabolite: Bupropion XL to Hydroxybupropion

Bupropion XL is converted to Hydroxybupropion by CYP2B6.

The pair

Bupropion XL — Longest-release NDRI; activating and usually sexually sparing.
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 XL and Hydroxybupropion.