Electronic cigarettes (ECIGs) are a class of products that use an electrically powered heating element to aerosolise a liquid for user inhalation. One subgroup of the class, ‘closed-system’ ECIGs, is sold ready-to-use and is constructed of component parts that cannot be modified readily and are filled with liquids that cannot be accessed easily. Another subgroup of the class, ‘open-system’ ECIGs, allows the user to modify virtually every component part and/or fill them with any liquid. Thus, although any given closed-system ECIG can be considered a single product, any given open-system ECIG must be considered to be a nearly infinite number of products, as the user controls the system’s electrical power, heating element characteristics and liquid constituents, which almost always include solvents like propylene glycol and vegetable glycerin, sweeteners and flavourants, and at least one psychoactive drug such as nicotine or tetrahydrocannabinol. This user control is important because every aspect of…