The ESTxENDS Trial- Substudy on the Effects of Using Electronic Nicotine Delivery Systems (ENDS) on Depression

--\> This is a substudy of the main ESTxENDS trial (NCT03589989). Depression outcomes should be considered secondary outcomes of the main smoking cessation outcome formulated in NCT03589989. Cigarette smoking is the leading cause of preventable death in Switzerland and still more than a quarter of the Swiss population smokes cigarettes. Smoking and depression are strongly associated. Individuals with depression are twice as likely to be smokers than persons without a depression. Studies have shown that attempts to quit tobacco smoking are more likely to fail for individuals with depression than without. Depressive symptoms are common in prolonged nicotine withdrawal and individuals with depression are more nicotine dependent and more likely to suffer depressive symptoms during nicotine withdrawal compared to smokers in the general population. Recently, electronic nicotine delivery systems (ENDS; also called vaporizer or electronic cigarette) have become popular with smokers who want to stop smoking or reduce their exposure to inhaled chemicals since ENDS use appears to be safer than tobacco smoking. ENDS with nicotine containing e-liquids may be effective in assisting with tobacco smoking cessation without suffering depressive symptoms. This study will therefore test the efficacy of ENDS for cigarette smoking cessation, the safety of ENDS on adverse events and the effect of ENDS on health-related outcomes such as depression and exposure to inhaled chemicals. For the main ESTxENDS trial (NCT03589989), cigarette smokers motivated to quit smoking cigarettes will be included. Participants in the intervention group will receive an ENDS and nicotine-containing e-liquids, which they will be allowed to use ad libitum. Additionally, they will receive smoking cessation counseling. Participants in the control group will receive smoking cessation counseling only. All participants will be followed over a 24-month period. Presence and severity of depression will be assessed u