Sales of e-cigarettes - particularly those with high nicotine content, similar to traditional cigarettes - skyrocketed in the US in 2017.
Proponents of e-cigarettes say this jump in sales should lead to a jump in those quitting smoking, pointing to some clinical trials as evidence of this.
Unfortunately, this didn't work out in practice.
According to a new paper in BMJ Tobacco Control, e-cigarettes were linked to lower success rates for those who tried to quit smoking, and they weren't any better at preventing relapses.
The study examines data from a US national long-term study on smoking.
The researchers looked specifically at data from 2017-2019, on 3578 established smokers who'd recently tried to quit and 1323 recent former smokers.
"We found little evidence that smokers took part in the 2017 surge in e-cigarette sales, which was associated with the introduction of the high-nicotine JUUL e-cigarette," says co-author Professor John Pierce, a researcher at UC San Diego and UC San Diego Moores Cancer Centre, US.
"This is the first survey in which e-cigarettes were less popular as a smoking cessation aid than FDA-approved pharmaceutical aids.
"Not only were e-cigarettes not as popular, but they were associated with less successful quitting."
In 2017, more than 12 per cent of recent quitters reported using e-cigarettes to quit - either by themselves or with other aids.
Almost two-thirds of respondents (64 per cent) didn't use anything.
By 2019, those who had used e-cigarettes were less likely to have successfully quit than those who'd gone cold-turkey - 10 per cent versus 19 per cent.
In this study, "successful quitting" was defined as having gone 12 months without using tobacco products.
However, the number of respondents who were using or planning to use e-cigarettes to quit had nearly doubled - to 22 per cent of all respondents.
The researchers stress that their study is observational - this data can't show that e-cigarettes are the cause for these failed quitting attempts.
But they do point out that their real-world data sits in contrast to other randomised clinical trials, which tend to slightly favour e-cigarettes over other quitting methods.
"RCTs [randomised clinical trials] are usually conducted under optimal conditions, which means that they may not translate to the effectiveness of the product in community settings," point out the authors in their paper.
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