Some studies have performed a cumulative risk assessment on tobacco smoke toxicants by computing a total MOE. A total or cumulative MOE can be calculated by building the reciprocal of the sum of the reciprocals of the single MOEs. The implementation of this method was not possible in the current study because the toxicants have different target organs and effect mechanisms that makes it impossible to estimate a meaningful total MOE for tobacco toxicants.

This study allows comparing the potential toxicological risk of several tobacco smoke constituents especially in comparison with nicotine by using the MOE approach. For that purpose toxicity data of nicotine has been used to estimate the MOE and the results were compared to MOEs of various other tobacco toxicants from the literature. Due to the fact that adequate toxicity threshold data were unavailable for nicotine, BMDL values had to be calculated from literature dose-response data by own modelling. BMDL10 values of nicotine were derived for five toxicological endpoints (Table 1). For four of those endpoints the margins of exposure were calculated (Fig. 1). All four computed MOE distributions of nicotine are below 10 and may be interpreted as indicating a very high risk of this compound for tobacco smokers.

The MOE for the endpoint “heart rate acceleration” is based on data from Lindgren et al.26 who investigated the effect of nicotine in humans after i.v. dosage. As this endpoint is the most sensitive effect, the MOE is the lowest (0.04). However, this may be an overestimation of the risk. The detected heart rate increase of approximately 7 beats/min was still within the normal range of intraday fluctuations27 and is therefore not necessarily an adverse effect. The Lindgren et al.26 data is also statistically questionable for dose-response assessment, because the standard deviations of the data points were not provided and had to be estimated (see data appendix). In addition, a discrepancy exists in the study regarding the exposure units. On the one hand, the unit of nicotine is specified in the unit ng/ml plasma. However identical numerical values are mentioned in the results section as being in the unit μg/kg bw, which is not mathematically possible. The corresponding author was not able to clarify the discrepancy in the data (Lindgren 2015, personal communication). For these reasons, we believe that the MOE data based on Lindgren et al.26 cannot be judged as reliable and should be carefully scrutinized prior to their use as the basis for risk management action. It must be mentioned that EFSA28 judged Lindgren et al.26 as a pivotal study, which was exclusively used as the basis for EFSA’s risk assessment, the rationale probably being to provide the most conservative assessment.

The second considered effect was “addiction” in humans based on data from Benowitz and Henningfield29. The derived average MOE is 0.2. This is clearly in the high risk range and biologically plausible as nicotine is known as the addictive principle in tobacco29,30. Nevertheless, we are reluctant to use this value for risk assessment, as “addiction” is a rather vague concept which changed its definition remarkably often over the past 50 years, even if only medical classifications are considered31,32,33.

The other two endpoints rely on animal bioassays. For the effect “changes in rat liver” (fatty change, focal necrosis and dark cell change) an average MOE of about 0.61 was calculated based on data of Yuen et al.10. This calculation may, however, underestimate the risk of nicotine due to the very short duration of the liver toxicity study of only 10 days.

Finally, according to the severity of the toxicological endpoint, the average MOE of 7.6 for animal mortality is the highest (data based on several acute studies on various animal species summarized in Lachenmeier and Rehm24).

For the toxicological endpoint “various symptoms of intoxication in children” based on data from Woolf et al.34 the MOE was not calculated. The authors believe that this acute dermal exposure study in children is not applicable for risk assessment of habitual smoking, because of the different exposure conditions and questionable transferability to adults.

With a MOE from below 1 up to 7.6, the risk of nicotine is in the same dimension as the tobacco (smoke) toxicants with the lowest MOE such as acrolein, formaldehyde and cadmium compounds, which are the tobacco toxicants with the highest concerns relating to adverse health effects. There appears to be a fundamental problem that nicotine has never before been included in any risk assessment on tobacco or tobacco smoke toxicants. There are numerous studies about toxicological investigations from the tobacco industry and other authors available. Those studies tried to identify the most important toxicants from tobacco and to correlate them to the various diseases caused by cigarette smoking. However, nicotine has not been evaluated in any of them5,6,13,35,36. Up to now nicotine has not been associated with carcinogenesis but the risk of long-term nicotine intake is more or less unknown and under-researched12. The correlation of the disease risk to smoking dose, the mode of action and the etiology of the disease pathologies are not well understood36. Pankow et al.37 suggested that only about 4% of the observed risk for lung cancer can be explained by tobacco smoke toxicants. Also, in this case nicotine has not been part of the research. Could nicotine therefore explain a large part of the remaining risk? Currently there is only limited evidence to corroborate this hypothesis. Some experimental studies of different laboratories show that nicotine might promote or increase the risk of cancer1,12. West et al.12 demonstrated that it can stimulate the growth of lung cancer cells and may contribute to apoptosis. In alcohol, it had also been postulated for a long time, that ingredients other than ethanol were the main carcinogens, before ethanol itself was found as the main causal agent38,39. The correlation between smoking cigarettes and cancer is well established2. However, nicotine is so far considered as an addictive substance in tobacco smoke but not as a carcinogen. Bavarva et al.40 investigated the genomic influence of nicotine and its genotoxic mechanism mediated through oxidative stress in a cell line experiment. The results indicate that nicotine exposure can adversely affect the human genome by inducing somatic mutations and may contribute to increased cancer incidence, characterizing nicotine as a carcinogen or mutagen.

In addition to the ones located in the brain, nicotine receptors are found throughout the body; for example, in muscle, endothelia, kidney and skin, in normal lung and in lung tumors. These receptors are involved in a number of cellular pathways of carcinogenesis. That provides some mechanistic plausibility to the hypothesis that nicotine may contribute to the carcinogenic process1. But the studies have limitations in replicating human exposure and human evidence is clearly lacking so far. More research is needed to clarify if nicotine may make the major contribution to the mechanistically unexplained adverse effects. For example, there is no study on cancer endpoints currently available that would allow to calculate a MOE for this effect. We can only speculate if the pyrolysis products have been a misleading focus for research whereas the actual carcinogen has been neglected. Nicotine has previously not been evaluated by IARC but is on the current list of IARC to be re-evaluated with high priority41.

Our research showing that nicotine is among the top risk compounds in tobacco smoke leads to the question why there are no nicotine reduced cigarettes available on the market. Possibly the tobacco industry would lose clients if the nicotine exposures falls below the threshold of addiction42,43. It is proven that nicotine-reduced cigarettes would be less addictive44,45. For example, in a 10 week longitudinal study, the subjects at first smoked their usual brand followed by different types of research cigarettes with progressively lower nicotine content, each smoked for 1 week. After 4 weeks, 25% of the participants had spontaneously quit smoking46. The results of a 6-week research show that nicotine-reduced cigarettes lower its exposure, dependence and the number of cigarettes smoked45. These findings suggest that reducing the nicotine content of cigarettes could facilitate to quit smoking and therefore increase public health45. In the USA nicotine reduced cigarettes is a topic of major concern for research and policy since the commencement of an act in 200942. Low-nicotine cigarettes have been and are on the US market43. However, they have not been successfully marketed owing to the issue of how much nicotine may be removed without affecting the taste. The tobacco industry did extensive research to determine the threshold of targeting nicotine dosing by cigarettes42. Benowitz and Henningfield29 derived a threshold level of nicotine in cigarettes from 5 mg or less to avert addiction. Instead Land et al.47 concluded that the tobacco manufacturers even increased the nicotine yield over the years.

In contrast, some studies showed that smoking nicotine reduced cigarettes leads to more intensive smoking to compensate for the necessity of nicotine48,49,50,51. For example, Ashton et al.48 suggested that smokers may compensate for about two-thirds of the difference in standard yields when switched from medium-nicotine to high or low-nicotine brands. The results show evidence of both upward and downward self-titration of nicotine intakes by smokers. This was confirmed by Gable52 who concluded that smokers tend to titrate to approximately 1 mg per cigarette by varying number and duration of puffs. Henningfield49 concluded that there are no health benefits on smoking cigarettes with lower (tar and) nicotine levels compared to “normal yields”. Woodward and Tunstall-Pedoe51 believe that in the case of deeper inhalation more carcinogenic pyrolysis products would be absorbed and concluded that not cigarettes with the lowest nicotine yields but those with low tar, low carbon monoxide and high nicotine yields appears to be the “most safe” cigarette. In contrast, Frenk and Dar53 do not confirm the “nicotine compensation hypothesis”. They are of the opinion that if nicotine is addictive in the same sense that heroin is, smokers would rapidly increase their dose (like heroin-dependent users do) by smoking more cigarettes or increasing number of puffs or deeper inhalation of the puffs or switching to cigarettes with a higher nicotine yield. But smokers do not. They arrive rapidly at their preferred number of cigarettes per day and the number remains stable for years. The conclusion of Frenk and Dar53 is that nicotine seems to be involved in a down-regulation mechanism but not in up-regulation. In fact light cigarettes may be smoked more intensely because of the sensory reward of their reduced tar and taste.

Another important aspect which needs to be considered is the fact that a high number of people use tobacco replacement or nicotine products over many years without getting cancer54, for instance pharmaceutical nicotine products for nicotine replacement therapy (NRT). NRT products such as nasal sprays, gums, tablets, lozenges and transdermal patches are marketed for helping people who want to stop smoking. The intended use of NRT products is the application for a fixed period of time. Only few users really quit smoking after this recommended time of 8–12 weeks. A lot of people do not stop the application after weeks but use these products over years some of them in addition to smoking. The US FDA does not have any significant safety concerns if smokers use nicotine replacement products in combination with another product or if they do not stop smoking completely before beginning to use smoking cessation products. NRTs do not appear to have a meaningful potential for abuse or dependence55. On the other hand, it may be difficult to monitor chronic risks such as cancer from over-the-counter products such as NRTs, and confounding to the co-consumption of NRT with smoked tobacco is difficult to exclude.

If we assume that nicotine alone or in the form of NRTs does not have any chronic effect, there could be more factors involved. One of those might be the type of nicotine exposure. The most precise difference in nicotine intake is the speed of effect. It takes only a few seconds for high doses of nicotine from a cigarette to reach the brain when inhaled. Medicinal products achieve lower levels over a period of minutes for products such as nasal spray or oral products and hours for transdermal patches22,56. Another essential point seems to be the complex mixture of nicotine and the other tobacco smoke toxicants. This is the exact difference from conventional cigarettes to electronic cigarettes (e-cigarettes) and NRTs. In conventional cigarettes, tobacco leaves are burned and nicotine is transferred besides toxic compounds produced during the combustion of tobacco such as carbon monoxide and nitrosamines from the solid phase to the aerosol. In e-cigarettes a nicotine solution is heated and nicotine is transferred from the liquid phase to the vapor phase. Because the harmful constituents of cigarette smoke are absent or significantly reduced in e-cigarette aerosols, smoking e-cigarettes is supposed to be less hazardous than smoking conventional cigarettes57,58. Hence, it may be speculated that nicotine and another compound in tobacco smoke may have cumulative effects.

In conclusion, all MOEs of nicotine in this study are less than 10 and within the range of very high risk. Owing to the lack of toxicological data particularly relating to cancer30, long term animal bioassay studies for nicotine are urgently necessary. In this context all kinds of possible nicotine exposures as well as certain co-exposures (tobacco smoke toxicants) and different nicotine concentrations related to a potential threshold of addiction need to be considered. The study from Lindgren et al.26 needs to be replicated. There is immediate need of action concerning the risk assessment of nicotine.