The demographic characteristics of the whole population as well as smoking variables among quitters are reported in Table 1. A total of 144 smokers quit smoking for at least 1 year. A greater proportion of slow metabolizers quit smoking (Fig. 1A), consistent with an increased likelihood of quitting by slow metabolizers relative to normal metabolizers (odds ratio (OR) 2.3, 95% confidence interval (CI) 1.1, 4.8; P=0.037) (Fig. 1B). There was no significant difference in the probability of quitting by intermediate nicotine metabolizers compared to normal nicotine metabolizers (OR 1.3, 95% CI 0.7, 2.2; P=0.436) (Fig. 1B), however a significant linear trend towards increased quitting with decreasing CYP2A6 metabolism (SM>IM>NM) was observed (OR for linear trend across CYP2A6 activity groups = 1.44, 95% CI 1.0, 2.0; P=0.034). In an exploratory analysis restricted to regular smokers (n=188, defined as monthly smokers), we observed a similar direction and size of effect, where slow metabolizers (n=12) were more likely to quit than normal metabolizers (n=154; OR 1.7, 95% CI 0.58, 4.74), although the smaller numbers reduced power.
