Cigarette smoking causes an estimated 120,000 deaths per year in the UK and costs the NHS about £2.6bn per year. Quitting smoking is therefore desirable but can be difficult even with professional support. A recent strategy for quitting is the use of e-cigarettes because they can be used to vary nicotine intake, their vapor does not contain known cancer-causing agents found in cigarette smoke, and their use closely mimics cigarette smoking behavior. However, these attractive features can lead to long-term use, and little is known about the resulting health outcomes because e-cigarettes have only recently become widely used. We plan to investigate these potential health outcomes by measuring the effects of e-cigarette use on DNA methylation in saliva and in blood. These effects will then be linked to health outcomes using Mendelian Randomization and publicly available genetic associations.