Champix is a prescription-only medicine containing the active ingredient varenicline, which can help you to quit smoking. Champix is a medicine specifically designed to help you stop smoking.It works by reducing the cravings and withdrawal symptoms associated with stopping smoking, making the process easier.hampix (varenicline) works by mimicking the effects of nicotine on the brain thus reducing withdrawal symptoms and cravings. Champix stimulates nicotinic receptors in the brain, which relieves the craving and withdrawal symptoms you can get when you stop smoking.