First used as a (rather ineffective) anti-depressant in the US, Zyban contains a substance which helps you fight cravings. Up to 30% of ex-smokers who took Zyban have stayed smoke-free. What's more, it helps you not put on weight as well. However, it can have side-effects such as insomnia, anxiety or irritability.
Champix works like nicotine, helps you overcome cravings and also blocks nicotine from acting on the receptors in your brain. This stops you from getting pleasure from smoking should you start again (though obviously smoking while you're on Champix isn't advised). According to several studies, Champix is more effective than Zyban: 44% of smokers don't start again after 12 weeks' use, compared with 30% on Zyban. But it can also have side-effects: nausea, insomnia and strange dreams.