Objective To examine what substances US youth vape.Methods Data come from Monitoring the Future, an annual, nationally representative survey of USA 12th-grade, 10th-grade and 8th-grade students. Respondents reported what substance they vaped the last time they used a vaporiser such as an e-cigarette.Results Among students who had ever used a vaporiser, 65â€”66% last used â€˜just flavouringâ€™ in 12th, in 10th and in 8th grade, more than all other responses combined. In all three grades, the percentage using â€˜just flavouringâ€™ was above 57% for males, females, African-Americans, Hispanics, Whites, and students both with and without a parent with a college degree. Nicotine use came in a distant second, at about 20% in 12th and 10th grade and 13% in 8th grade. Taking into account youth who vaped nicotine at last use increases national estimates of tobacco/nicotine prevalence in the past 30â€…days by 24â€”38% above and beyond cigarette smoking, which is substantial but far less than estimates that assume all vaporiser users inhale nicotine.Conclusions These results challenge the common assumption that all vaporiser users inhale nicotine. They (a) call into question the designation of vaporisers and e-cigarettes as ENDS (â€˜Electronic Nicotine Delivery Systemâ€™), (b) suggest that the recent rise in adolescent vaporiser use does not necessarily indicate a nicotine epidemic, and (c) indicate that vaporiser users can be candidates for primary prevention programmes. Finally, the results suggest the importance of developing different rationales for the regulation of vaporiser devices as compared to the regulation of substances marketed for vaporiser use.