A recent series of studies indicate that varenicline, recently approved by the FDA, may be more effective than bupropion in helping smokers quit and promoting longer periods of abstinence. Each drug had related side effects: approximately one-third of varenicline users complained of nausea, and nearly a quarter of bupropion users experienced insomnia. Although the majority of trial patients didn't quit smoking permanently, regardless of the drugs prescribed, varenicline users had higher immediate and long-term abstinence rates than those taking bupropion or placebo.

Source