Ecigs are almost exclusively used as a safer and cheaper alternative to smoking, though some may try them as a route to smoking cessation.
When considering this particular issue, it is first necessary to decide exactly what is meant by 'smoking cessation': is it quitting smoking, or total cessation of all inhaled products? People may interpret the term differently. Some feel that when no cigarettes are consumed, a person is then an ex-smoker. Some think that use of alternatives or harm reduction products (especially those that accurately replicate the original) means the activity has not actually ceased. There are also legal implications for use of certain of these terms, which may affect their use - that is to say, they cannot be used when they should be, or there is a reluctance to use them.
There is also a clear unwillingness to use terms relating to medical products when discussing a consumer product that has no relation to any medical function. There are complications when a consumer harm reduction product is classified as or used as a medical product, and in practice it has proven impossible to test and trial it as a medical product; this fact has caused much confusion.
The result is that there is no clear agreement on 'ecigs for smoking cessation' and in the end it can only be a personal choice as to precisely how they are described or used. Even the use of established terms is controversial, since the terms 'electronic cigarette' or 'e-cigarette' are strictly marketing terms, as the product itself bears no relation to the original - unless it is specifically manufactured (with huge functional negatives) to replicate a cigarette for the purpose of attracting new customers, in which case it simply looks like the original article. There is no possible functional comparison because the system is smoke-free, tobacco-free and combustion-free; it is a battery-powered device producing water-based vapour. Inhalation of the aerosol produced has no relation to inhalation of smoke.
There are a fixed number of outcomes with EV use (electric/electronic vapourisers), and people will always follow one of these paths.
1. The most common result for smokers trying and using an EV is that the smoker will fail to quit, and will relapse to smoking.
There is an argument that this depends principally on the availability of mentoring. It is phrased like this: "If smokers buy an EV it will most likely be a mini (aka a cigalike, or 1st-gen product), and they will have no access to mentoring. Therefore they will not know about any of the options. Only about 10% to 20% of smokers will succeed in switching via this route (at 12 months). At the other end of the scale, when a smoker is closely mentored by an expert family member or work colleague, they will succeed in switching in about 75% of cases".
What we do know is that free choice and good advice is critical for success - mentoring is always crucial for any type of smoking cessation, and without it the chances of success are drastically reduced. Mentoring is especially important for new EV users, since only a few of the many thousands of options will be optimal for any given smoker; the principal function of mentoring is to find a working solution for the individual.
We can deduce from R West's figures that most UK smokers trying or using EVs will fail and revert to smoking, although it is impossible to say what the exact figure is because derivation is a longitudinal (ongoing) process, and there is not enough data yet. We also know that success rates would be higher if more mentoring were to be available.
West reports that of the 2.1 million UK vapers at the time of his last survey, Q1 2014, one-third had become ex-smokers. However it is an over-simplification to say that therefore two-thirds will continue to smoke, because vapers may stay as dual-users, or progress over time from dual-use to either relapse to smoking only or to exclusive vaping; and no one knows yet what the average time points are for any subsequent action.
(Note: a few will quit smoking on Day 1 of vaping, but without any better information this is assumed to be a small minority.)
2. Almost all EV (electric/electronic vapouriser) use is dual-use initially, because few smokers switch completely to EVs on Day 1 (we assume).
Therefore the very first conclusion we can draw from this is that dual-use is a good thing, because it is the main route from smoking to exclusive vaping. Secondly, any vaping means less smoking; and since NICE tell us that smoking less is a form of harm reduction*, their opinion appears to be that every cigarette smoked counts; therefore a cigarette not smoked is a benefit.
* Not everyone would agree on this point, but that is a different matter.
There are many things we don't know about dual-use that would be interesting to know:
One thing we do know about it, in the UK, is that dual-use is the single most important route to smoking cessation, of all routes, bar none. This is because:
So dual-use of EVs and cigarettes is now the principal route to smoking cessation and smoking prevalence reduction in the UK.
This may or may not be true in other countries, but without the data it is difficult to make any realistic evaluation.
3. Some people cease smoking and become vapers.
A percentage of smokers who try an EV will become exclusive vapers at some point. The ASH UK / Prof R West research reports that one-third (33%) if those who use EVs become exclusive vapers or quit totally. The last survey revealed that, of the 2.1 million smokers who had used an EV, some 700,000 had ceased use of tobacco cigarettes. This figure is more reliable for use as a final number than the other side of the numbers ("what percentage will fail?"), for the reasons given above.
33% sounds a lot if you come to this situation from scratch, and have no knowledge of the potential of EVs compared to other methods of smoking cessation. In fact it's a median-scale figure:
When you know this, a 33% result looks fairly reasonable, as it reflects the mix of personal situations of the millions of smokers trying ecigs - anything between 25% and 35% could probably be expected as a final success rate at present (although this is not likely to relate to the eventual population groupings after some decades, since other factors will operate). As with any smoking cessation method, mentoring is critical - the success rate is at least halved if no mentoring is available. Today, the internet provides some measure of mentor provision for those without other options - forums provide support for lone vapers. Some idea of the importance of this resource is shown by the tens of millions of posts on the EV forums, a level of engagement several orders of magnitude greater than for any other smoking alternative or cessation method.
Also, we have nothing to compare it with, of course - there are no statistics from any other country. In the entire world, it appears that only Prof West is collecting and releasing independent statistics that cannot be interfered with by those with an agenda. This is important: the US CDC for example doesn't collect or publish such stats, in order to deliberately prevent detailed knowledge of ST and EV use - but if it did, they would certainly not be trustworthy.
4. Total cessation
Cessation of use of all inhalation products can also occur. There are two ways this normally takes place:
It is currently impossible to give any stats for this result, since even R West's figures do not tell us this, at least so far.
We know that around 7% of vapers at any given time are in the zero-nic group. This is because most surveys give this figure or one close to it, and the outliers tend to balance out. Zero-nic vapers are mostly those who have transitioned into this group by gradually reducing the nicotine strength of the refills they use. Most vapers reduce the strength of their refills over time, and reduce their total nicotine intake, by either reducing the nic strength or reducing the amount vaped, or both.
This is because nicotine does not create tolerance. If this is difficult to comprehend: it is first necessary to separate the effect of one ingredient from a list of 9,600 (the total number of compounds in tobacco identified so far). The effect of one compound on its own is not the same as when that compound is mixed with about 9,600 others, especially when some of them are powerful potentiators for that compound (synergens, if you prefer). Pure nicotine creates neither tolerance nor dependence; these are created by the mixture in tobacco.
The starting dose eventually becomes too strong for most vapers, and they have to reduce it in order to avoid increasing effects of over-consumption. In other words, nicotine is too strong once smoking recedes into the past; this is because smoking creates tolerance but pure nicotine doesn't, and because smoking "blunts" the effect of nicotine, as researchers discovered and stated decades ago.
Some vapers reduce their nic intake to the extent they transition into the zero-nic group, perhaps because they hope to quit at some point. Perhaps they have no intention to quit at all; but as we have seen, EVs are unique among smoking-related products as they can result in unmotivated quitting (of all/any type), a phenomenon completely unknown for any other product .
5. Ecig use by non-smokers
In a country where a THR product is freely available, smoking prevalence reduces and consumption of the THR product increases. We already have the Swedish model for this: Sweden is the world leader in the reduction of smoking prevalence as male smoking prevalence falls at 1% per year, and will be just 5% by around 2016.
However it appears that a set percentage of the population in western developed countries need dietary nicotine supplementation, and that this is around 20 or 25%. It doesn't seem to matter what the preferences, laws or other influences are: about a quarter of the adult population will choose to consume nicotine additional to normal dietary sources. Therefore if smoking reduces, or is somehow eliminated (impossible of course due to the black market, which can easily supply the population with whatever it needs), then people will still be consuming their nicotine in some other form. In Sweden we see that the local population prefers Snus to smoking - but there are no less snusers than there would otherwise be smokers.
We can infer that if smoking falls below about 20% of the adult population, other nicotine consumption methods will take its place. This is also clear from the US experience, where Rodu tells us that there are 8 million ST users, which would represent a significant percentage of the tobacco-using population (which is why US smoking prevalence figures are affected signicantly by the ST-using population - there is at least a 3% reduction due to this factor but it may be higher).
There are said to be about 42 million US smokers and 8 million ST (oral tobacco) users, of which some may be duail-users. There are, in addition, several million EV users (and again, some will be dual-users). Because we know from the UK that there is a 33%-66% split in EV users between ex-smokers and dual users, we can at least say that some EV users will be ex-smokers, although it may be wrong to assume the proportions are the same in other countries. The same sort of factors probably apply to US smokers / ST users. All we can state reliably is that US tobacco or tobacco alternatives usage prevalence is certainly going to be significantly higher than the current circa 18% given for US smoking prevalence.
Thus, if people don't take up smoking, they will take up something else that provides the same end result - the final figure for nicotine supplementation consumers appears to always equal or exceed 20% of the adult population in western countries. This means as smoking falls, EV use will rise, and people who would previously have started smoking will instead start vaping.
This process reduces smoking-related mortality and morbidity toward zero since modern THR products have no clinically-visible morbidity-creation ability. Statistically, mortality and morbidity from modern THR products is invisible, or not reliably identifiable, so that the overall effect is as if the nicotine-consuming proportion of the population had entirely ceased all tobacco-related activities (they are non-smokers as far as morbidity is concerned). R West has stated that mortality from EV use may eventually be seen to be around 0.03% per year (about 300 deaths per million EV users) at the most.
People may raise concerns about future use of ecigs by non-smokers; but this is an inevitable effect of the reduction of smoking, and has no clinical implications in any case.
The argument is, essentially, whether or not people should consume things like coffee. In the modern urban environment, it seems difficult to prevent that, assuming either that (a) consumption has any clinically-measurable value - which it doesn't; or (b) that anyone has a right for some reason to stop people doing things that benefit them and harm no one else - which they don't.
'Benefit' is far too difficult a concept for analysis by any branch of public health, which is blind to life enhancement, economics, or any other factor relevant to consumer choice.
Clinical trials to 'measure the efficacy' of EVs are worthless. It is like trying to measure the efficacy of tea, but restricting the subjects to loose green tea of one brand.
The entire concept is ridiculous in the first place, in any case. Firstly, 'efficacy' of a consumer product is an irrelevant concept - people will use it if the complex mix of benefits to them, personally, attracts them; secondly, most potential tea drinkers won't like the single, non-variable product offered and will 'fail'.
Clinical trial results of EV efficacy are therefore irrelevant. It would be necessary to multiply the usual success rate reported in CTs by a factor of around 3 to reach the worst-case real-world result. This is because CTs restrict the hardware, refill base type, refill flavour, and nicotine strength; and more importantly, restrict access to mentoring. They have to, since the mentor's first task is to find a solution that roughly fits the subject; the second task is to refine that choice until it is perfect. Since mentoring is about finding a working solution as much as anything else, it cannot be allowed in CTs, as no variation from the trialled product is permitted.
This means that most potentially 'successful' candidates will 'fail'. It looks to be around two-thirds of them, measured against the known lone-vaper success rate; and about four-fifths, measured against the fully-mentored anecdotal success rate.
The best way and the only way to see if EVs are successful is to measure their success in the real world. Currently, only R West / ASH UK have any procedure for doing this. Lab tests, of all types, are a disaster area for researchers and EVs.
There are some related points of interest:
 EVs are unique among smoking cessation methods and alternative products in that they can cause a smoker to quit who had no intention of quitting. If a number of smokers with no intention to quit are given EVs, then some (the percentage is currently unknown) will eventually quit smoking, normally after a period of dual-use, and transition completely to vaping (EV use). Certainly no licensed pharmaceutical causes measurable levels of unmotivated quitting or 'quitting by accident', as EVs do.
 It is unclear at this time if, like EVs, Snus can result in unmotivated smoking cessation or unmotivated total cessation, as these issues do not appear to be reported on.