Electronic cigarettes (EC) are a developing technology aiming to provide nicotine without the harmful chemicals produced by tobacco combustion (Etter 2012). EC have the potential to generate a substantial public health benefit if there is a switch from smoking to EC use on a population scale (Public Health England 2015; RCoP 2016). So far, however, only a minority of smokers who try EC progress to the full switch from smoking to vaping (Douptcheva et al. 2013; Kralikova et al. 2013; Farsalinos et al. 2016). This suggests that the currently available EC products do not yet match combustible tobacco closely enough in providing smokers with what they want from their cigarettes. The nicotine delivery profile is likely to play a major role (Caldwell et al. 2012; McRobbie et al. 2010).

The parameters of nicotine delivery that are likely to be important for smokers include the overall nicotine dose and the speed of nicotine absorption (Schroeder and Hoffman 2014). Several studies compared nicotine yields from cigarettes and from EC using puffing machines (Cobb et al. 2010; Farsalinos et al. 2013; Goniewicz et al. 2014; Goniewicz et al. 2013; McAuley et al. 2012; Pellegrino et al. 2012; Trehy et al. 2011; Westenberger 2009). Earlier EC were shown to deliver much less nicotine than cigarettes, but recent devices used at high power setting have improved nicotine delivery (Farsalinos et al. 2016). Machine yields, however, may not correspond with the actual delivery of nicotine into the blood stream of users and the method provides no information about the speed with which nicotine is absorbed. Some studies measured levels of cotinine, a metabolite of nicotine, in vapers, but this is more difficult to interpret because it is not known what proportion of cotinine is derived from nicotine that has been swallowed; and cotinine levels also provide no data on speed of nicotine delivery.

Some data comparing nicotine concentrations in blood samples from smokers and vapers exist as well. In smokers with no experience of EC use, first generation ‘cig-a-like’ EC delivered nicotine very slowly and at low concentrations (Bullen et al. 2010; Vansickel and Eissenberg 2012a; Vansickel et al. 2012b) suggesting buccal rather than pulmonary absorption. There is some evidence that experience with EC improves nicotine intake (Hajek et al. 2014) and so studies with experienced vapers using their EC ad-lib are probably more informative. In a study with experienced vapers who were, however, still prescribed a puffing schedule, nicotine concentrations reached 7 ng/ml 10 min after a 10-puff bout (Dawkins and Corcoran 2014). For comparison, plasma nicotine concentrations from a single cigarette average 15–20 ng/ml (Benowitz et al. 2006). Ad-lib vaping for an hour generated nicotine concentrations of up to 14 ng/ml in one study (Dawkins and Corcoran 2014) and 16 ng/ml in another (Vansickel and Eissenberg 2012a). In smokers with no experience with EC and a fairly intensive puffing schedule prescribed, 5 min after initiation, venous blood nicotine levels achieved with 8, 18 and 36 mg liquid were 9, 13 and 17 ng/ml (Lopez et al. 2015) while in an identical experiment with experienced vapers, the levels were 18, 26 and 36 ng/ml (Ramôa et al. 2015).

Apart from user experience and puffing characteristics, the type of EC is likely to play a major role. More advanced refillable EC products with stronger batteries delivered nicotine more efficiently than a ‘cig-a-like’ EC in one study (Farsalinos et al. 2014). Cigarettes typically reach time of maximal nicotine concentration (T max ) within a few minutes (Digard et al. 2013) while early first-generation EC achieved T max after 20 min (Bullen et al. 2010). Experienced vapers using their own EC, mostly tank systems, though again with a prescribed puffing schedule, averaged maximum plasma nicotine concentration (C max ) of only 8 ng/ml, but the T max was 5 min (St Helen et al. 2015). Even higher levels were achieved in a recent study where experienced vapers used an advanced vaping device with 24 mg/ml e-liquid (44 ng/ml after 1 h) (Dawkins et al. 2016).

Up to now, little is known about differences in nicotine delivery between individual EC brands. Among nicotine replacement treatments, those that deliver nicotine faster are more likely to be used long-term (Hajek et al. 2007; Sutherland et al. 1992). It is likely that compared to EC with low and slow nicotine delivery, devices with a faster and higher nicotine delivery will appeal to smokers more and will have a better potential to replace cigarettes and assist with smoking cessation. Other EC characteristics such as taste, ease of use, puff resistance, vapour volume and handling characteristics (and of course cost, as well as product marketing) are likely to be important too, but it can be expected that the nicotine delivery profile will be paramount. One sign of this is that nicotine containing EC dominate the market with nicotine-free models hardly used despite being otherwise equivalent and widely available (Etter 2012). Nicotine delivery, however, may need to fit into a relatively narrow range at both ends. Very high nicotine concentration liquids are also rarely used.

EC technology is evolving and market forces are likely to steer product development to features that appeal to smokers and increase the rate of adoption, but the process could be slow. Data are needed that monitor pharmacokinetic (PK) characteristics of different EC brands to provide information that could guide smokers faced with the wide range of different EC products, inform the choice of EC brands for studies of the potential of EC in smoking cessation, and guide further product development.

In the first study of this type, we tested PK profiles of eight common EC brands, together with conventional cigarettes and with vapers’ own devices. We also adjusted the common methodology in this field to our particular aim. Most studies assessing nicotine intake from alternative nicotine delivery products take the first post-baseline sample 5 min after the initial nicotine intake. We started 2 min after the first puff and followed the changes in nicotine absorption in 2-min intervals to allow for more accurate comparison with smoking and to assess whether speed of nicotine absorption suggest a buccal or a much faster pulmonary route. We also asked experienced vapers to use each device after overnight abstinence ad lib. EC studies up to now have typically used prescribed puffing schedules that do not allow for individual adjustments smokers make when using different products and may not correspond to ‘real-life’ levels of nicotine that vapers obtain from vaping.