Plasma concentrations are summarized in Table 2. In all subjects, the mean plasma concentration of D2-cotinine increases two-fold from 30 min to 150 min, but remains unchanged from 150 to 240 min. These data are consistent with the rapid conversion of nicotine to cotinine and the relatively long half-life of cotinine. Mean D2-cotinine and D2 trans 3′-hydroxycotinine concentrations of female subjects were significantly higher than those of male subjects at all time-points but only the difference in D2 trans 3′-hydroxycotinine remained statistically significant after controlling for weight (p=6.6×10−5, data not shown). D2-cotinine plasma concentrations 30 min following D2-nicotine oral administration were significantly higher in current smokers than in current non-smokers (p=0.0015). This difference also remained significant after excluding carriers of the *2, *4,*9 and *12 alleles (p=0.002). The difference was more pronounced among women (p=0.0005) than among men (p=0.15) and remained significant after correcting for weight. D2-nicotine, D2-cotinine, and D2-trans 3′-hydroxycotinine concentrations did not differ significantly between current smokers and non-current smokers after 150 or 240 minutes.
