It's a novel medicine for smoking cessation. Cigarettes just aren't as satisfying when you take this twice a day. From the FDA website:
"The U.S. Food and Drug Administration (FDA) today approved Chantix (varenicline tartrate) tablets, to help cigarette smokers stop smoking. The active ingredient in Chantix, varenicline tartrate, is a new molecular entity that received a priority FDA review because of its significant potential benefit to public health.
Chantix acts at sites in the brain affected by nicotine and may help those who wish to give up smoking in two ways: by providing some nicotine effects to ease the withdrawal symptoms and by blocking the effects of nicotine from cigarettes if they resume smoking...
The approved course of Chantix treatment is 12 weeks. Patients who successfully quit smoking during Chantix treatment may continue with an additional 12 weeks of Chantix treatment to further increase the likelihood of long-term smoking cessation.
In clinical trials, the most common adverse effects of Chantix were nausea, headache, vomiting, flatulence (gas), insomnia, abnormal dreams, and dysgeusia (change in taste perception).