An additional factor in the persistence of smoking behaviors is the difficulty in quitting. Although many methods have been developed that improve quit rates, none is effective in all smokers (Lerman et al, 2007). Among these methods are several pharmacological agents, including nicotine replacement therapy (NRT), bupropion, and varenicline. Bupropion is a dopamine/norepinephrine reuptake inhibitor that also acts as a nicotinic receptor antagonist (Warner and Shoaib, 2005); varenicline is a partial agonist of the α4β2 nAChR subtype (Coe et al, 2005). The determinants of successful smoking cessation, like nicotine dependence itself, are likely to be diverse. The genetic components of successful smoking cessation are less well understood than nicotine dependence itself, although, like nicotine dependence, a significant proportion (∼50%) of the likelihood of quitting is genetic in origin (Broms et al, 2006; Lessov et al, 2004; Xian et al, 2003), suggesting that specific genetic risk factors could be identified. Indeed, the chromosome 15q25 locus described above has been associated with successful quitting in pregnant women (Freathy et al, 2009). However, many of the genetic loci affecting quitting are likely to
