Varenicline drug molecule. Computer model of varenicline, a smoking cessation aid sold under the trade names Chantix and Champix, manufactured by Pfizer. Varenicline is a central nervous system drug called a nicotinic acetylchlorine receptor partial antagonist that stimulates the nicotine receptors in the brain. It also prevents the nicotine from acting on the receptors itself and therefore stops cigarettes from having a rewarding and enjoyable effect. Atoms are colour-coded: carbon (yellow), hydrogen (white) and nitrogen (blue).