Associations between the use of snus (moist tobacco) or electronic cigarettes and tobacco smoking

A systematic review

Summary and main findings

Main findings

  • Experimentation with snus or e-cigarettes may be a predictor for subsequent initiation of cigarette smoking. The certainty of evidence was higher for e-cigarette data than for snus.
  • Due to scarce material for snus, and very heterogenous results in studies of e-cigarettes, we could not find quality evidence for associations between either use of snus or use of e-cigarettes, and a change in smoking behaviour among individuals using smoking tobacco.

Background

According to a report from The Swedish Council for Information on Alcohol and Other Drugs (CAN) on tobacco habits in Sweden 2003–2018, about a quarter of the Swedish population use tobacco, either daily or occasionally [1]. Snus is more commonly used by men, where 25% reported having used snus in the previous month. The corresponding figure for women is 6%. E-cigarettes (electronic cigarettes) are used by 1 to 2% of the adult population, and among second year high school students, 6% state that they have used e-cigarettes in the past month [2,3]. Like snus, e-cigarettes are more commonly used by men.

This systematic review investigates possible associations between using snus or e-cigarettes, and smoking tobacco. Subgroups of men, women, children <18 yrs., and adults were analysed. This field of research is rapidly expanding, when it comes to e-cigarettes, while the number of studies about snus is scarce. This systematic review provides a current review of the research.

Aim

The aim of this systematic review is to assess possible associations between snus or e-cigarette use, and subsequent change in cigarette smoking behaviour (initiation of smoking/quit smoking/increased smoking /decreased smoking) at follow up.

The two main review questions:

  1. Is ever use of snus or e-cigarettes a predictor for later initiation of cigarette smoking, among individuals that do not smoke at the beginning of the study period.
  2. Is there evidence for associations between use of snus or use of e-cigarettes, and a change in smoking behaviour, measured as increase, decrease or quitting, among people smoking tobacco at the beginning of the study period.

Method

The literature was evaluated using a priori established protocols, and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. The certainty of evidence was assessed with GRADE [4,5,6,7], as very low (⊕◯◯◯), low (⊕⊕◯◯), moderate (⊕⊕⊕◯) or high (⊕⊕⊕⊕).

Inclusion criteria

The study question formulated according to the PICO format:

Population*

  • Research question 1: General population samples of non-smokers, any age.
  • Research question 2: General population samples of smokers, any age.

Intervention/Exposure

Self-reported current or ever use of snus or e-cigarettes, with or without nicotine

Comparison

Self-reported non-use of snus or e-cigarettes, with or without nicotine, all through the study period.

Outcome

  • Research question 1: Self-reported ever or current use of combustible tobacco products at follow up.
  • Research question 2: Self-reported non-use, non-use for at least 30 days, increased use or decreased use of combustible tobacco products at follow up.

For research questions about snus, only studies investigating individuals living in the Nordic countries were included.

Study design

Studies reporting empirical results, with either longitudinal observational design with a minimum of 3 months of follow-up, or Randomised Control Studies (RCT); including general population samples; allowing for the comparison between users and non-users of snus or e-cigarettes.

Language

English, Swedish, Norwegian, Danish

Search period

From 1998 to 2019. Final search November 11, 2019.

Databases searched

PubMed incl. Medline, EMBASE, Cochrane Library, Scopus, PubMed Health, NICE evidence search, PROSPERO, CRD, and PsycInfo

Client/patient involvement

No

Results

Literature search

There are 73 primary research articles included in the different analyses of this systematic review (Figur 1). Most of them are investigating research questions about e-cigarettes. Eight are investigating snus.

Figure 1 Flow chart.

Summary of results

An overview and summary of the main results is presented in Table 1 and 2.

Snus

It is possible that experimentation with snus may be a predictor for later initiation of cigarette smoking (certainty of evidence ⊕⊕◯◯). However, association between experimentation with snus and current use of cigarettes was not found (certainty of evidence ⊕◯◯◯).

Due to scarce material it was not possible to draw any conclusions about associations between use of snus, and quitting smoking (certainty of evidence ⊕◯◯◯), or between use of snus and increased smoking (certainty of evidence ⊕◯◯◯). With the present inclusion criteria, no studies investigating associations between use of snus and quitting smoking for at least 30 days, or between use of snus and decreased smoking, were found.

E-cigarettes

It is probable that experimentation with e-cigarettes may be a predictor for later initiation of cigarette smoking (certainty of evidence ⊕⊕⊕◯). The certainty of evidence was higher among young individuals (<18 years) (certainty of evidence ⊕⊕⊕◯) but could also be found among adults (certainty of evidence ⊕⊕◯◯). Association between experimentation with e-cigarettes and current use of cigarettes was also found (certainty of evidence ⊕⊕⊕◯).

Due to very variable results in the included studies that investigated e-cigarettes and changes in smoking behaviour, it was not possible to draw any conclusions about possible associations between e-cigarettes and quit smoking, quit smoking for at least 30 days, decreased smoking or increased smoking (certainty of evidence for all four outcomes ⊕◯◯◯).

Table 1 Overview of main findings. The direction of association is shown if the certainty of evidence according to GRADE was (⊕⊕◯◯) or higher. Arrow pointing upwards: association between snus or e-cigarettes and a higher incidence of the outcome. Arrow pointing downwards: association between snus or e-cigarettes and a lower incidence of the outcome.

Outcomes that are non-relevant to a specific research question are denoted by NA.

Table 1 Overview of main findings.

Table 2 Summary of main findings.
aMaterial with several deficits and limitations.
bThe confidence interval for one or several of the included studies include the value for no association.
cThe analysis is based on a limited amount of studies or participants.
dOne or several of the included studies include only one subcategory of the general population e.g. only men or only individuals younger than 18 years.
eSome of the included studies show a positive association, while others show a negative one.
fThe time to follow-up is long, or varies a lot between studies, which makes the association between exposure and outcome less clear.
gThe confidence interval of the meta-analysis (unadjusted/adjusted/ continuous/ dichotomous) include the value for no association.
hUnadjusted data or data adjusted for confounders is lacking.
iLimitations in transferability for results from clinical study of smoking cessation to behaviour in the general population (refers to, among other things, differences in population, availability of intervention, comparison alternatives).
jVariations in the way to define the outcome.
kThe adjusted and unadjusted analyses differ both regarding which studies that are included in the meta-analyses, and their results.
* Also studies with data presented in forms that could not be transformed to either odds ratios or mean differences, were considered during the grading of evidence. These studies were not included in the meta-analyses but were incorporated narratively when appropriate, as follows:1
1Continuous abstinence rate measured between 9–24 weeks [8]. Smoking cessation was achieved by 28.0 percent of the participants in the group who were allocated to nicotine chewing gum and by 21.3 percent of the participants allocated to e-cigarettes. No statistically significant difference was seen between the groups.
2 Unadjusted data: One study (persons 12–17 years) indicates an association between e-cigarette use and increased use of smoking tobacco (frequency), the association is not statistically significant [9]. One study (adults) indicates an association between e-cigarette use and reduced use of smoking tobacco (quantity), the connection is statistically significant [10]. Two studies (adults) indicate an association between e-cigarette use and reduced use of smoking tobacco (quantity), the associations are not statistically significant [11,12].
3Adjusted data: Two studies show an association between e-cigarette use and reduced use of smoking tobacco (frequency and quantity) [13] or (quantity) [10] , the associations are statistically significant. Three studies indicate a link between e-cigarette use and reduced use of smoking tobacco (quantity), the links are not statistically significant [11,12,14]. One study indicates an association between e-cigarette use and increased use of smoking tobacco (frequency and quantity), the link is statistically significant [15]. One study indicates a link between e-cigarette use and increased use of smoking tobacco (frequency) [16,17], the link is not statistically significant.
Outcome Study design
Number of studies (with unadjusted + adjusted data)
Number of participants
Results, Odds ratio (95% CI) GRADE Comment
Snus and initiation of tobacco smoking Longitudinal observational design
5 (5+3)
23 472
Unadjusted:
2.09 (1.57 to 2.79)
Adjusted:
2.48 (1.79 to 3.44)
⊕⊕◯◯a,b,c,d It is possible that Swedish snuff may be a predictor for later initiation of cigarette smoking
Snus and current tobacco smoking Longitudinal observational design
3 (3+3)
3443
Unadjusted: 2.16 (1.08 to 4.31)
Adjusted:
2.48 (1.79 to 3.44)
⊕◯◯◯a,b,c,d,f  Not enough data to draw conclusions about associations
Snus and quitting smoking Longitudinal observational design
2 (2+0)
6350
Unadjusted:
1.98 (1.72 to 2.28)
Adjusted:
No data
⊕◯◯◯a,c,d,f,h Not enough data to draw conclusions about associations
Snus and increased tobacco smoking Longitudinal observational design
1 (0+1)
No information
Unadjusted:
No data
Adjusted:
6.21 (3.20 to 12.05)
⊕◯◯◯a,c,d,h Not enough data to draw conclusions about associations
E-cigarettes and initiation of tobacco smoking Longitudinal observational design
22 (17+20)
89 076
Unadjusted: 4.68 (3.64 to 6.02)
Adjusted:
3.37 (2.68 to 4.24)
⊕⊕⊕◯a It is probable that e-cigarettes may be a predictor for later initiation of cigarette smoking
E-cigarettes and current tobacco smoking Longitudinal observational design
10 (7+9)
39 086
Unadjusted:
3.51 (2.87 till 4.29)
Adjusted:
3.89 (2.16 till 7.00)
⊕⊕⊕◯a It is probable that e-cigarettes may be a predictor for later current cigarette smoking
E-cigarettes and quitting smoking* Longitudinal observational design 28 (18+14)
39 147
RCT
8
3202
Unadjusted:
0.99 (0.78 till 1.33)
Adjusted:
0.95 (0.70 till 1.28)
RCT:
1.78 (1.41 till 2.25)
Data not included in the meta-analyses: 1
⊕◯◯◯a,b,e,g,i Data to diverse to draw conclusions about associations
E-cigarettes and quitting smoking at least 30 days* Longitudinal observational design
17 (9+9)
13 588
RCT
4
2368
Unadjusted:
0.96 (0.77 till 1.19)
Adjusted:
0.86 (0.59 till 1.25)
RCT:
2.04 (1.51 till 2.77)
Data not included in the meta-analyses:1
⊕◯◯◯a,b,c,e,g,i,k Data to diverse to draw conclusions about associations
E-cigarettes and decreased tobacco smoking* Longitudinal observational design
13 (7+12)
14 817
RCT
7
2851
Unadjusted:
1.22 (0.89 till 1.66)
Adjusted: 1.46 (1.03 till 2.08)
RCT:
OR:1.79 (1.26 till 2.55)
Mean Difference: 
1.08 (–0.38 till 2.54)
Data not included in the meta-analyses: 2,3
⊕◯◯◯a,b,e,g,i,j Data to diverse to draw conclusions about associations
E-cigarettes and increased tobacco smoking* Longitudinal observational design
11 (6+9)
13 286
RCT
3
891
Unadjusted:
1.79 (1.40 till 2.29)
Adjusted:
1.91 (1.36 till 2.69)
RCT:
OR: No data
Mean Difference:
1.08 (–0.38 till 2.54)
Data not included in the meta-analyses: 2,3
⊕◯◯◯a,b,c,e,g,i,j Data to diverse to draw conclusions about associations

Discussion

This systematic review analyses possible associations between snus, e-cigarettes and the use of smoking tobacco in contexts transferable to Swedish conditions and to the population in general. For snus, only studies investigating use of snus in the Nordic countries are included. Studies on e-cigarettes have been included regardless of e-cigarette type or country of origin. No studies on e-cigarettes conducted in a Nordic context were found.

Our results show that there may be an association between using snus or e-cigarettes and later initiation of tobacco smoking. The certainty of evidence was higher for e-cigarette data than for snus.

It was however not possible to assess whether there is an association between use of snus or use of e-cigarettes and changed smoking habits among individuals using smoking tobacco. In the case of snus, this was due to the low number of primary studies investigating the possible association between snus and changes in smoking behaviour. In the case of e-cigarettes, it was due to very diverse results in the different primary studies included in the analyses. However, both the ongoing intensive research on e-cigarettes, and new studies about snus may change the current state of knowledge.

Based on the results of this systematic review, it is not possible to determine whether the associations found in the material are causal, or mainly statistical relationships. In most of the included studies, it is possible that confounders affect the outcome. There may be underlying differences between the comparison groups that affect both the use of snus or e-cigarettes and the use of smoking tobacco. We have therefore not assessed the strength of the associations found, but only if there is an association or not. A non-significant result means that an association could not be statistically confirmed. It should not be interpreted as an evidence of no association. Based on the results of the systematic review, it is not possible to determine what the smoking habits of the population would have looked like if snus or e-cigarettes had not been on the market.

The purpose of this project has been to extract new knowledge. The report does not contain any proposals for changes in regulations or application of practice.

Project group

Experts

  • Louise Adermark, Associate Professor, Sahlgrenska Academy, Gothenburg University, Sweden
  • Maria Rosaria Galanti, Professor, Karolinska Institute, Sweden
  • Hans Gilljam, Professor, Karolinska Institute, Sweden
  • Linnéa Hedman, Associate Professor, Umeå University, Sweden

SBU

  • Charlotta Ryk, Project Manager
  • Elisabeth Gustafsson, Project Administrator
  • Martin Norman, Assistant Project Manager (from September 2019)
  • Hanna Olofsson, Information Specialist
  • Karin Stenström, Assistant Project Manager (until June 2019)
  • Mikael Nilsson, Head of Department

External reviewers

  • Charlotta Holm Pisinger, Professor, Rigshospitalet, Copenhagen, Denmark
  • Karl Erik Lund, Professor, Norweigan Institute of Public Health, NIPH Norway
  • Arja Rimpelä, Professor, Tampere University, School of Health Sciences, Finland

References

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  2. Centralförbundet för alkohol- och narkotikaupplysning, CAN. Monitormätningarna. Tillgänglig från: https://www.can.se/undersokningar/monitormatningarna/. Nedladdad 2020-02-10.
  3. Centralförbundet för alkohol och narkotikaupplysning, CAN. Skolelevers drogvanor 2019. CAN-rapport 187. Red. Englund. A. Tillgänglig från: https://www.can.se/app/uploads/2020/01/can-rapport-187-skolelevers-drogvanor-2019.pdf. Nedladdad 2020-04–29.
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Full report in Swedish

The full report in Swedish "Samband mellan snus, e-cigaretter och tobaksrökning"

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SBU Policy support identifies and presents available scientific evidence to support policy and decision making, including the development of national guidelines, at other government agencies. In consultation with professional experts, SBU staff generates supporting documentation to address the various questions that have been posed.

Published: 6/15/2020
Report no: 312
Registration no: SBU 2018/431
https://www.sbu.se/312e