Not everything that is safe to swallow is safe to inhale. What this means is that certain e-liquids might have more complex chemical compositions in order to achieve flavors such as vanilla, caramel or strawberries.
Sweet or fruity flavoring seems to be more dangerous that your average tobacco or menthol flavors, but toxicity is hard to predict – in fact, most manufacturers never list the products that go into achieving a particular flavor, but only list the regular chemical ingredients required to make a vaporizer-adequate e-liquid. It’s an intelligent decision since the flavor is often what makes a particular e-liquid manufacturer stand out in the crowded market, but it is also a source of frustration for researchers.
Nothing of this means, of course, that inhaling e-liquids are more dangerous than smoking cigarettes, however it would be useful to understand in full what exactly we are consuming to avoid possible health hazards.