E-cigarettes or vaping devices are battery devices that aerosolise a liquid for inhalation. The liquids used in e-cigarettes vary and contain a range of flavouring chemicals, solvents and many contain nicotine. They have been enthusiastically marketed via social media as reduced harm products to help people quit cigarette smoking and uptake has been rapid with an estimated 86 million e-cigarette users globally.1 The safety of e-cigarettes has been downplayed by manufacturers. There are hundreds of harmful chemicals in e-cigarette aerosols, and users are inhaling listed poisons and harmful by-products such as acetaldehyde, formaldehyde and acrolein.2 While the levels of chemicals are generally low in e-cigarette vapour, vaping at high frequency means e-cigarette use cannot be assumed to be harmless.3 As e-cigarette use has only been popular for a short time, there is currently insufficient evidence linking e-cigarette use to long-term respiratory disease.4 However, identified chemicals that damage the respiratory system and new evidence on the risk of acute inflammation and decreased lung function, suggest there is potential for e-cigarettes to cause diseases like chronic obstructive pulmonary disease, chronic bronchitis, emphysema, asthma and cancers in the respiratory tract.4 Nicotine is very addictive and use of nicotine e-cigarettes is high and concerning among youth and adults who have never smoked. In the Asia Pacific, there are over 1 billion young people aged 10–24 years making up 60% of the world's youth population.5 An estimated 15.7 million people were vaping in the Asia Pacific in 2018 but this may be as high as 23.3 million now if patterns persisted since 2018.1 Of major concern is the moderate-to-strong evidence showing uptake of tobacco cigarette smoking among e-cigarette users who were never smokers (called the gateway effect) that was 21.5% in a recent meta-analysis.2 High and growing e-cigarette use threatens to reverse tobacco control measures and legislation and progress towards the World Health Organization's Framework Convention for Tobacco Control (FCTC). Each year tobacco smoking is responsible for direct and indirect economic losses worth US$247,866M and US$175,915M in Southeast Asia and the Western Pacific, respectively.6 Although the economic costs of e-cigarette use to society are unknown, with the high uptake of e-cigarette among never smoking youth and adults, future costs will also be high. If e-cigarette use cause harms amounting to just 10% of the total cost of tobacco smoking, it will add US$42 Billion in economic losses to the region. Lower workforce productivity alone magnifies the economic burden that is highest in those who can least afford it, and is very high in low and middle income nations.7 Tobacco smoking rates among men in low- and middle-income Asia-Pacific countries are highest in Indonesia (54.3%), Lao People's Democratic Republic and China and lowest (<20%) in Australia, India and New Zealand. However, India has high rates of smokeless tobacco use of 18.2% in 2018.7 From large studies on the US e-cigarette retail market, findings indicate that e-cigarettes are economic substitutes for cigarettes, that any taxation on e-cigarettes is fully passed on to consumers and youth demand for e-cigarettes is sensitive to price changes.8 However, these findings may have low relevance for the Asia-Pacific region. There is a strong economic case for regulating the e-cigarette industry to protect consumers due to information failure of both the safety of e-cigarette products and the addictiveness of e-cigarette use, as well as persistent interference from the tobacco industry.9 If these trends continue unrestrained, massive social, public health and personal costs will result in future years.10 Many countries have begun to impose regulations to curb use of e-cigarettes in youth. Regulations for the sales or use of e-cigarettes, and/or their e-liquid contents, in the Asia-Pacific are diverse. All e-cigarettes are completely banned in Singapore, Thailand, Bhutan, India, Sri Lanka and Timor Leste (as of December 2021).11 Nicotine-containing e-cigarettes are banned in Japan, Cambodia and Australia (except for the state of Western Australia that bans all e-cigarette sales), although because most e-cigarette devices are unlabelled it is impossible for authorities to police this and over 80% of devices have been found to contain nicotine in laboratory studies.2 Regulations about where sales occur, minimum age limits, limits to nicotine concentration or flavours/contents, are used in China, Fiji, Indonesia, Lao PDR, Malaysia, New Zealand, Philippines, South Korea and Vietnam.11 With emerging evidence on the questionable safety and long-term effects of e-cigarettes,4 law makers are deliberating on how to manage the duality of allowing nicotine e-cigarette products to be accessed by people who smoke but want to quit versus stemming the sharp uptick among adolescent and youth never smokers. Population simulation modelling is an approach to assess hypothetical policy scenarios for their costs and/or health effects. There are three such modelling studies since 2019 in the Western Pacific that have evaluated the long-term population consequences of e-cigarette use in the wider context of smoking policies including one each from Australia,12 New Zealand,13, 14 and Singapore.15 All three countries have very advanced tobacco control policies in implementing the FCTC, yet have diverse e-cigarette policies. At the one extreme, Singapore completely bans e-cigarettes and it is a crime to possess them. New Zealand takes a laissez-faire approach legalizing sales to adults while Australia, as of May 2023, will ban non-nicotine e-cigarettes but continues to permit nicotine e-cigarettes only via a medical prescription for smoking cessation. Despite some commonalities in these three models, more differences exist such as the use of open15 versus closed cohorts,12-14 different key inputs (e.g., the relative risk of disease for e-cigarettes to cigarettes was 5%,12 10%15 or 37%14), definitions of smokers/vapers (e.g., daily,12-14 last 30 days15), ages (12+,15 18+12-14) and other structural differences (e.g., permanent smoking cessation at 2 years,12 dual smoking included15). Unsurprisingly, their conclusions about whether or not to support restricted versus unrestricted population access to e-cigarettes arise from the assumptions about model input values (e.g., safety, behaviours) as well as the regulation types and the underlying model structures. Further work is needed to strengthen the evidence base and modelling methodologies to inform practice but until then, stronger regulations on e-cigarette supply and use should remain at the forefront of public health policy to guard against the significant uncertainty around e-cigarette effects. Open access publishing facilitated by Queensland University of Technology, as part of the Wiley - Queensland University of Technology agreement via the Council of Australian University Librarians. None declared.