While the nicotine regulation model helps explain the behavior of daily smokers (DS), this model seems less suitable for explaining the behavior of nondaily or intermittent smokers (ITS). By current Cisplatin Sigma estimates, a quarter to a third of adult smokers in United States are ITS (Centers for Disease Control and Prevention [CDC], 2008a, 2008b; Substance Abuse and Mental Health Services Administration [SAMHSA], 2009; though see Hassmiller, Warner, Mendez, Levy, & Romano, 2003; Wortley, Husten, Trosclair, & Chrismon, 2003). The prevalence of ITS has increased substantially in recent years (e.g., CDC, 2003; though see CDC, 2007), perhaps partly due to increasingly stringent tobacco-control measures (Shiffman, 2009b). Similar patterns are evident in Europe (Korhonen, Broms, Levalahti, Koskenvuo, & Kaprio, 2009; Lindstrom & Ostergren, 2001) and elsewhere (e.

g., Asia, Latin America; World Health Organization [WHO], 2007). We currently know little about ITS smoking behavior and dependence. In a recent study (Shiffman, Tindle, et al., 2012), ITS reported abstaining on over one-third of days and abstaining for periods of approximately five consecutive days, on average, with some reporting abstaining for 10 or more consecutive days during a 2-month period. On the days that they did smoke, ITS consumed approximately 4�C5 cigarettes/day. These data suggest that ITS are less dependent than DS. This is consistent with findings from prior studies on ITS (Gilpin, Cavin, & Pierce, 1997; Hennrikus, Jeffery, & Lando, 1996) and on chippers��light smokers, most of whom smoke daily (Shiffman & Sayette, 2005).

If maintaining nicotine levels above a certain threshold��or indeed, at any level above zero��is essential to dependence, then ITS may not show any dependence at all, as they are unable to maintain nicotine levels above zero while skipping days of smoking. On the other hand, DiFranza and Wellman (2005) have argued that dependence is often evident in novice smokers, does not require nicotine maintenance, and can be seen even after smoking just a single cigarette (DiFranza et al., 2000; Scragg, Laugesen, Wellman, & DiFranza, 2000). Importantly, ITS apparently find it difficult to quit. In a national survey, Tindle & Shiffman (2011) found that many ITS had made attempts to quit smoking (more so than DS; also see Shiffman, Tindle, et al.

, 2012) but had very poor success rates: only about 20% of ITS who made quit efforts were abstinent for 90 days at the time of the survey. Further, one in eight ITS used medication in their quit attempt; given that medication use is not common (Shiffman, Brockwell, Pillitteri, & Gitchell, 2008), and tends to be adopted by more dependent smokers (Shiffman, Brockwell, et al., 2008), who anticipate Entinostat greater difficulty quitting, this also suggests that ITS have difficulty quitting. (Cooper et al.