Hydroxybupropion (code name BW 306U), or 6-hydroxybupropion, is the major active metabolite of the antidepressant and smoking cessation drug bupropion.[1] It is formed from bupropion by the liver enzyme CYP2B6 during first-pass metabolism.[1] With oral bupropion treatment, hydroxybupropion is present in plasma at area under the curve concentrations that are as many as 16–20 times greater than those of bupropion itself,[1][2] demonstrating extensive conversion of bupropion into hydroxybupropion in humans.[1] As such, hydroxybupropion is likely to play a very important role in the effects of oral bupropion, which could accurately be thought of as functioning largely as a prodrug to hydroxybupropion.[1]