More than twice as many youth in the United States currently use 2 or more tobacco products than cigarettes alone. Continued monitoring of tobacco use patterns is warranted, especially for e-cigarettes. Youth rates of multiple product use involving combustible products underscore needs for research assessing potential harms associated with these patterns.

http://pediatrics.aappublications.org/content/early/2015/01/28/peds.2014-3202.full.pdf