Electronic cigarettes (e-cigarettes) have proved very popular and a meteoric rise in their usage is currently under way. People purchase them as an aid to giving up smoking, to reduce cigarette consumption, to minimise withdrawal symptoms in occupational environments that ban smoking, and in order to continue smoking with decreased health risks. Although the safety and impact on health of electronic cigarettes, especially after long-term use, has not been evaluated, they are generally considered to be far safer alternatives to cigarette smoke. Electronic cigarettes do not generate polycyclic aromatic hydrocarbons, a potent class of carcinogenic chemicals generated during the combustion of tobacco and making important contribution to the cigarette-induced cancer. However, carcinogenic tobacco-specific nitrosamines have been encountered in e-cigarettes being detected in some nicotine cartridges as contaminants, albeit at very low concentrations in comparison with tobacco smoke. Consequently, it is imperative to ascertain the toxicity risk (if any) of consuming nicotine intake through electronic cigarettes. This European Commission funded study will monitor levels of carcinogenic tobacco-specific nitrosamines in urine of heavy smokers who give up smoking and completely transition to e-cigarette use for a period of 4 weeks. Levels of other compounds which are known to be associated with smoking toxicity, such as DNA adducts and DNA methylation, will also be monitored in biological fluids of these subjects. Finally, cigarette craving, mood, anxiety, social anxiety, well-being status and stress hormones will be measured in smokers transitioning to e-cigarettes for 4 weeks to assess the psychological effect of the transition. The results from the study will provide important information on the safety and effectiveness of e-cigarettes for smoking cessation which investigators anticipate to drive policy.