In May 1997, the FDA approved bupropion as a smoking cessation aid, marketed under the name Zyban rather than how it was already known: the antidepressant Wellbutrin. While bupropion is approved as the only nicotine-free first-line therapy to help a person quit smoking, the FDA isn't able to explain how it actually helps (similarly, there is no clear explanation of how bupropion treats depression, either). Unlike most antidepressants, bupropion has little to no effect on the body's noradrenaline and/or serotonin reuptake receptors nor on its muscarinic, histaminic and/or a-adrenergic receptors; the theory of its effectiveness is that it interacts with the brain's reward and pleasure system.
Bupropion is just one of two antidepressants used to help patients quit smoking, though; nortriptyline (Pamelor, Aventyl) is a tricyclic antidepressant considered as effective as bupropion when it comes to helping patients kick the smoking habit. Nortriptyline, though, is used off-label because the FDA hasn't yet approved it as a smoking cessation aid.