and levels of cigarette consumption. In a sample of over 1000 young adults, Haberstick et al. (2007) found that time to first cigarette was the most informative measure of heritable factors from the FTND score. Our results complement these findings by illustrating that necessity to smoke measured by time to first cigarette after waking at least partly drives the association of the CYP2A6 metabolism metric and nicotine dependence in young adult daily smokers. Although the physiologic mechanism underlying this association remains unknown, slow metabolizer daily smokers likely have more consistent nicotine levels throughout the day compared to fast metabolizer daily smokers, which may contribute to a feeling of greater necessity to smoke in the morning when nicotine levels are low.
