The highly controversial e-cigarette industry has generated considerable policy debate and mixed regulatory responses worldwide. Surprisingly, an issue that has been largely ignored is the categorisation of e-cigarettes as substitutes or (dynamic) complements for conventional smoking. We conduct an online survey of US participants finding that 37% of e-cigarette users view them primarily as complementary. We use this result along-side publicly available data to calibrate a cost-benefit analysis, estimating that complementarity reduces the potential costsavings of e-cigarettes by as much as 57% (or $3.3-4.9bn p.a.) relative to case with zero complementarity.