They can affect your brain chemistry.
Doctors prescribe Chantix for people who are likely to have trouble breaking the habit of lighting up. Chantix blocks the brain's ability to enjoy the surge of chemicals, so that if you light up while you're trying to quit, your brain will not reward you. This drug doesn't contain nicotine, and it doesn't act as a replacement for nicotine in the brain.
According to Pfizer, the manufacturer of Chantix, 44 percent of the drug's users in a study had quit within three months of starting treatment, compared to 18 percent who had quit using a placebo.
Zyban, an antidepressant, minimizes the effects of dopamine in the brain and mimics some of the effects of nicotine. GlaxoSmithKline, the manufacturer of the drug, reported that 36 percent of patients who received a dose of 300 milligrams per day had reported quitting within seven weeks, compared to 17 percent of patients in the trial who used a placebo.
Unlike with Chantix, some studies have found that Zyban mimics some of the effects of nicotine in the brain.